Toolbar: Only show on WikiText pages

The toolbar is highly specific to Wikitext anyway. It was already not
added to JS and CSS subpages, but still visible on site css/js pages
and on Lua pages. This was inconsistent.

Ideally at some point, we might want a 'toolbar api' where you can
configure a 'toolbar' per content model, but let's first make it
consistent and then iterate from there when required.

Bug: 29908
Change-Id: If0853e9faf640f93b7030c4937137149d3f08cf5
Merge "Toolbar: Only show on WikiText pages"