Use dynamic gradient based on fee levels on mempool blocks.

fixes #64

frontend/src/app/components/mempool-blocks/mempool-blocks.component.html

@@ -1,7 +1,7 @@
 <div class="mempool-blocks-container">
   <div class="flashing">
     <div *ngFor="let projectedBlock of mempoolBlocks; let i = index; trackBy: trackByFn">
-      <div class="bitcoin-block text-center mempool-block" id="mempool-block-{{ i }}" [ngStyle]="getStyleForMempoolBlockAtIndex(i)">
+      <div class="bitcoin-block text-center mempool-block" id="mempool-block-{{ i }}" [ngStyle]="mempoolBlockStyles[i]">
         <a [routerLink]="['/mempool-block/' | relativeUrl, i]" class="blockLink">&nbsp;</a>
         <div class="block-body" *ngIf="mempoolBlocks?.length">
           <div class="fees">