fix: Make icons unselectable (#478)

No longer select icons when marking text, this prevents copying the
icon.

Resolves #474

Author: MoritzWeber0

assets/sass/fonts.scss

@@ -145,4 +145,5 @@ button {
   text-decoration: none;
   width: 1em;
   height: 1em;
+  user-select: none;
 }