Change mempool project license to GNU Affero General Public License v3

The files contained in this source code repository are currently
released as "open source" under the terms of the [MIT license](https://opensource.org/licenses/MIT),
with the additional protections of the [Commons Clause License Condition v1](https://commonsclause.com/),
in order to restrict monetization of the mempool app, with the
intended result of mempool remaining free software (as in free
beer) for the Bitcoin community.

However, because of the Commons Clause being added to our license terms,
The Mempool Open Source Project cannot formally be considered as [Free
and open-source software (FOSS)](https://en.wikipedia.org/wiki/Free_and_open-source_software),
according to the generally accepted definition of ["What is free
software?"](https://www.gnu.org/philosophy/free-sw.en.html), and this
has drawn some criticism from the Bitcoin open source community.

Therefore, in order to make The Mempool Open Source Project truly FOSS,
I hereby propose to change our project's license to the [GNU Affero
General Public License v3](https://www.gnu.org/licenses/agpl-3.0.html),
which seems most suitable for a web application. While this won't keep
the mempool free as in free beer, it will keep mempool free as in free
speech, which is what actually matters.

Here is [the TL;DR of AGPLv3](https://tldrlegal.com/license/gnu-affero-general-public-license-v3-(agpl-3.0)):
> The AGPL license differs from the other GNU licenses in that it was
> built for network software. You can distribute modified versions if
> you keep track of the changes and the date you made them. As per usual
> with GNU licenses, you must license derivatives under AGPL. It
> provides the same restrictions and freedoms as the GPLv3 but with an
> additional clause which makes it so that source code must be
> distributed along with web publication. Since web sites and services
> are never distributed in the traditional sense, the AGPL is the GPL of
> the web.

Could all contributors to The Mempool Open Source Project please ACK
this PR?  @softsimon @wiz @bguillaumat @TechMiX @junderw @andrerfneves
@pox @jambolo @6102bitcoin @devinbileck @fiatjaf @timlucmiptev @Czino
@lucasmoten

LICENSE

@@ -1,35 +1,15 @@
-MIT License with Commons Clause License Condition v1.0
+The Mempool Open Source Project
+Copyright (c) 2019-2021 The Mempool Open Source Project Developers
 
-Copyright (c) 2019-2020 The Mempool Open Source Project
+This program is free software: you can redistribute it and/or modify it
+under the terms of the GNU Affero General Public License as published by
+the Free Software Foundation, either version 3 of the License, or (at your
+option) any later version.
 
-Permission is hereby granted, free of charge, to any person obtaining a copy
-of this software and associated documentation files (the "Software"), to deal
-in the Software without restriction, including without limitation the rights
-to use, copy, modify, merge, publish, distribute, and/or sublicense
-copies of the Software, and to permit persons to whom the Software is
-furnished to do so, subject to the following conditions:
+This program is distributed in the hope that it will be useful, but
+WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
+or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU Affero General Public
+License for more details.
 
-The above copyright notice and this permission notice shall be included in all
-copies or substantial portions of the Software.
-
-THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
-IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
-FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
-AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
-LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
-OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
-SOFTWARE.
-
-Commons Clause License Condition v1.0
-
-Without limiting other conditions in the License, the grant of rights under
-the License will not include, and the License does not grant to you, the
-right to Sell the Software.
-
-For purposes of the foregoing, “Sell” means practicing any or all of the
-rights granted to you under the License to provide to third parties, for a
-fee or other consideration (including without limitation fees for hosting or
-consulting/ support services related to the Software), a product or service
-whose value derives, entirely or substantially, from the functionality of
-the Software. Any license notice or attribution required by the License must
-also include this Commons Cause License Condition notice.
+You should have received a copy of the GNU Affero General Public License
+along with this program.  If not, see <http://www.gnu.org/licenses/>.