Don't allow edits or suggested edits to change moderator-only tags #1299
Description
A non-moderator with the Edit ability (or post owner) who tries to edit a post to modify moderator-only tags gets an error message about the tag restriction. A user proposing an edit to add one, on the other hand, can make that tag change -- but then non-moderators who would otherwise be able to approve the edit cannot, so the suggested edit is in limbo until a moderator handles it. In a related bug, a direct edit to remove a moderator-only tag succeeds and leaves no history on the post. -> Spun off in #1641
Can we intercept changes to restricted tags when the edit or suggested edit is submitted and complain? We could, optionally, add a line to the error message directing the user to flag for changes to mod-only tags.