This patch makes Codemirror on VE independent of the beta feature,
making it always load.
This is a first step to graduating CodeMirror out of beta.
Bug: T191923
Change-Id: Ide794e8f986d3f0455ff282819c71d9144dd75db
Long-term todo:
* Performance will be poor on large pages due
to using a auto-height textarea which CodeMirror
doesn't optimise.
Change-Id: I16598fcdbeee51e6fae88376ec81f1c8552b383d