UI/UX - New component for difficult adjustment. (#602)

* Add next difficulty blocks.
Add next difficulty target date.
Add next difficulty total progress.
Add ajustment difficulty avg min per block.

* Fix typo.

* Trigger difficulty calculation every 5 seconds.

* Add rxjs timer to difficultyEpoch.

* Fix pipe.

* Fix small bar position.

* Change i18n strings.

* Fix typo.

* Add time-until component.

* Speed up difficultyEpoch timer to 1000 ms.

* Fix values to 2 decimal places.

* Add title to fee and difficulty adjustment cards.

* Add title outside the card.

* Fix title to center position.

* Add other titles.

* Add new transalations strings.
Refactor time span component.

* Fix difficulty adjustment i18n string.
Fix duplicated i18n strings.

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

@@ -27,7 +27,7 @@
           <ng-template #mergedBlock>
             <div class="time-difference">
               <b>(<ng-container *ngTemplateOutlet="blocksPlural; context: {$implicit: projectedBlock.blockVSize / 1000000 | ceil }"></ng-container>)</b>
-              <ng-template #blocksPlural let-i i18n="shared.blocks">{{ i }} blocks</ng-template>
+              <ng-template #blocksPlural let-i i18n="shared.blocks">{{ i }} <span class="shared-block">blocks</span></ng-template>
             </div>
           </ng-template>
         </div>