Persistent disabling of CodeEditor

This makes it so that if you do not have WikiEditor enabled, you also
will not have CodeEditor enabled. In the feedback we have seen so far,
this seems to be desirable for those users (bug 55936).

Also people were not able with the cookie pref for the editor, because
it can easily expire. This introduces a hidden pref, controlled by the
enable/disable CodeEditor button, to preserve this value instead (bug
46779).

It also gives a more reliable way to detect if the editor is enabled or
disabled. mw.user.options.get( 'usebetatoolbar' ) and
mw.user.options.get( 'usecodeeditor' ) combined should be able to convey
this information to something like WikEd (bug 62250).

Bug: 46779
Bug: 62250
Bug: 55936
Change-Id: I4639f68c00a2b9183a6f89b5e00983c07a8592a2
Merge "Persistent disabling of CodeEditor"