Fix content property in SVG font icons

In commit 4d49ec8ef we replaced an `@extend .fa-` clause with a
`content: fa-content()` clause.

With the `@extend` clause, the `content:` property appeared wherever the
`.fa-` selector was defined, so we later overwrote it in our `%svg-icon`
selector, which was defined later in the generated CSS.

Defining the property with `content: fa-content()`, on the other hand,
caused the `content:` property to appear wherever we used the mixin with
`@include has-fa-icon`. That meant our `%svg-icon` selector would appear
before it, and would not overwrite it.

We could modify a few things and make the code more complicate in order
to avoid that. In this case, however, it's easier to add an `!important`
flag; after all, it is indeed important that SVG icons have no content
so screen readers don't try to announce illegible characters.

app/assets/stylesheets/mixins/icons.scss

@@ -7,7 +7,7 @@
 %svg-icon {
   @supports (mask-image: url()) {
     background: currentcolor;
-    content: "";
+    content: "" !important;
     height: 1em;
     mask-repeat: no-repeat;
     mask-size: 100% 100%;