Scenario: User A creates User:B/foo with a content model of CSS. The page can now only be edited by User:B and people with the editusercss permission (sysops typically). We should not allow users to change the content model of a page if they will be unable to edit it afterwards.

Possible approaches:

1. Make permission status separate from content model of a page (only protect if it ends in .css or .js, regardless of content model). Patch is https://gerrit.wikimedia.org/r/#/c/196982/ but @Jackmcbarn doesn't like that and proposed
2. Check the permissions of the page with the new content model, and if the user can edit that, then allow them to make the change. We need to override Title::mContentModel to do this, which is a bit hacky, but works. I'll also submit a patch for this.