Add "Downloaded" timestamp column to completed downloads list

Display the completion time for each download in the done list.
The backend already stores a nanosecond timestamp on DownloadInfo;                                     this wires it up to the frontend using Angular's DatePipe.

ui/src/app/app.ts

@@ -1,4 +1,4 @@
-import { AsyncPipe, KeyValuePipe } from '@angular/common';
+import { AsyncPipe, DatePipe, KeyValuePipe } from '@angular/common';
 import { HttpClient } from '@angular/common/http';
 import { AfterViewInit, Component, ElementRef, viewChild, inject, OnInit } from '@angular/core';
 import { Observable, map, distinctUntilChanged } from 'rxjs';
@@ -21,6 +21,7 @@ import { MasterCheckboxComponent , SlaveCheckboxComponent} from './components/';
         FormsModule,
         KeyValuePipe,
         AsyncPipe,
+        DatePipe,
         FontAwesomeModule,
         NgbModule,
         NgSelectModule,