Fix flash of bad ve-edit tab on non-wikitext page in enabled namespacea

E.g. when viewing User:Example/common.js there was previously
both Edit and Edit source (or Create and Create source) and the
client-side JavaScript then removed the ve-edit tab after checking
the content model for 've.isAvailable'.

Change-Id: Ic2daeefa439648ef0602f8606773655cf9ab188a