Ensure users are able to edit the page after changing the content model

Ensure users are able to edit the page after changing the content model

It is possible for page restrictions to be dependent upon the content
model a page. The best example of this is user JavaScript and CSS
subpages. This adds a Title::setContentModel() function which allows
mocking a Title's content model for the purpose of permission checks.

EditPage and Special:ChangeContentModel were updated to ensure the user
can edit the page with the newly proposed content model before making
the change.

Title::$mContentModel was made private to make sure nothing else mucks
around with it. There were no uses outside of Title anyways.

Bug: T145316
Change-Id: I28c46f408cadf65ed1868401cd00dc15e5acd2fe