Add button to add a repository when none are configured

Signed-off-by: trivernis <trivernis@protonmail.com>

mediarepo-ui/src/app/components/core/repositories-tab/repositories-tab.component.html

@@ -7,6 +7,10 @@
             <app-repository-card (openEvent)="this.onOpenRepository($event)"
                                  [repository]="repository"></app-repository-card>
         </div>
+        <app-middle-centered *ngIf="this.repositories.length === 0" class="add-repository-prompt">
+            <h1>There are no repositories yet. You can create a repository or add an existing one.</h1>
+            <button (click)="this.openAddRepositoryDialog()" color="primary" mat-flat-button>Add Repository</button>
+        </app-middle-centered>
     </div>
 </div>
 <div *ngIf="selectedRepository" class="repo-details">

mediarepo-ui/src/app/components/core/repositories-tab/repositories-tab.component.scss

@@ -31,3 +31,11 @@ app-repository-card {
 app-repository-details-view, .repo-details {
     height: 100%;
 }
+
+.add-repository-prompt {
+    button {
+        font-size: 1.5em;
+        padding: 0.5em 1em;
+        border-radius: 0.5em;
+    }
+}