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.
- 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
- 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.