Grant 'editcontentmodel' to '*' for MassMessageContentHandlerTest

Change-Id: Ibcb5371ee633951886e61888c912f88df0d45dd4
Implement AJAX warnings for CreateMassMessageList

In Special:CreateMassMessageList, the title is checked to see if it
already exists using AJAX, and a warning is given if so.

The source is also checked using AJAX to make sure it exists and has
content model of wikitext or MassMessageListContent.

Finally, implements progressive disclosure for the Source textbox
in a way that does not break if Javascript is disabled.

Bug: T72523
Bug: T72471
Bug: T72520
Change-Id: I3f4bcd39eb937f2c5f51662267e60d2b5f4c48a1