Replace footer and modal icons with SVG (#13245)

- Replace two font icons with SVG in the footer
- Replace icons in delete modals with SVG
- Horizontally center message text in modals

web_src/less/_base.less

@@ -679,6 +679,23 @@ code,
       }
     }
   }
+
+  /* modal svg icons, copied from fomantic except width and height */
+  &.icon.header svg {
+    width: 3em;
+    height: 3em;
+    float: none;
+    display: block;
+    line-height: 1;
+    padding: 0;
+    margin: 0 auto .5rem;
+    opacity: 1;
+  }
+
+  /* center text in fomantic modal dialogs */
+  &.modal > .content {
+    text-align: center;
+  }
 }
 
 .file-comment {