Set glitchy elephant friend variant in initial state. #294.

Also lay some groundwork needed to interactively change the
glitchfriend.

The codebase uses "elefriend" because it's shorter and didn't require me
to realign the actions in actions/compose.js.  Same idea, though.

app/javascript/flavours/glitch/styles/components/drawer.scss

@@ -66,8 +66,8 @@
     .mastodon {
       flex: 1;
 
-      @for $i from 1 through 3 {
-        &.mbstobon-bg-#{$i} {
+      @for $i from 0 through 2 {
+        &.mbstobon-#{$i} {
           background: url("~flavours/glitch/images/mbstobon-ui-#{$i}.png") no-repeat left bottom / contain;
         }
       }