views: factor out FontAwesome icons.

Factor out FontAwesome icons into a set of helpers. This is so that it's
easier to keep track of which icons we're using and easier to change
icons globally.

app/javascript/src/styles/common/main_layout.scss

@@ -5,12 +5,6 @@ div#page {
   margin: 0 20px;
   padding: 0 10px;
 
-  aside#sidebar {
-    #options-box i.fa-bookmark {
-      margin-right: 0.25em;
-    }
-  }
-
   aside#sidebar > section {
     margin-bottom: 1em;
   }

app/javascript/src/styles/specific/posts.scss

@@ -79,7 +79,7 @@ div#c-posts {
     }
   }
 
-  #remove-fav-button i.fa-heart {
+  form#remove-fav-button button:not([disabled]) i.icon {
     color: var(--remove-favorite-button);
   }
 
@@ -364,7 +364,7 @@ div#c-posts, div#c-uploads {
       display: inline-block;
     }
 
-    i.fa-external-link-alt {
+    i#open-edit-dialog {
       font-size: var(--text-xs);
     }