Fill out main content, align footer with bottom

Make the app root fill at least the full height of the viewport, and
make the main content grow to fill the empty space - aligning the footer
with the bottom of the page at all times.

ui/src/app/app.component.sass

@@ -93,22 +93,22 @@ td
 .d-flex.my-3
   margin-top: 1rem
   margin-bottom: 1rem
- 
+
 .modal.fade.show
   background-color: rgba(0, 0, 0, 0.5)
 
 .modal-header
   border-bottom: 1px solid #eee
-  
+
 .modal-body
   textarea.form-control
     resize: vertical
- 
+
 .add-url
   display: inline-flex
   align-items: center
   justify-content: center
- 
+
   .spinner-border
     margin-right: 0.5rem
 
@@ -127,6 +127,14 @@ td
                 overflow: visible
                 text-overflow: ellipsis
 
+app-root
+  display: flex
+  flex-direction: column
+  min-height: 100vh
+
+main
+  flex-grow: 1
+
 .footer
   width: 100%
   padding: 10px 0