Use ~ notation in components for image references

This uses (more or less) absolute references to external assets, which
allows imported stylesheets (like components.scss) to work from other
locations that may not have the expected path structure (e.g. the win95
theme).

app/javascript/styles/mastodon/components.scss

@@ -1758,7 +1758,7 @@
   position: absolute;
   top: 0;
   left: 0;
-  background: lighten($ui-base-color, 13%) url('../images/wave-drawer.png') no-repeat bottom / 100% auto;
+  background: lighten($ui-base-color, 13%) url('~images/wave-drawer.png') no-repeat bottom / 100% auto;
   box-sizing: border-box;
   padding: 0;
   display: flex;
@@ -1773,7 +1773,7 @@
   }
 
   > .mastodon {
-    background: url('../images/mastodon-drawer.png') no-repeat left bottom / contain;
+    background: url('~images/mastodon-drawer.png') no-repeat left bottom / contain;
     flex: 1;
   }
 }