Editing schemas should not be possible if:
- the user is blocked from this page
- the user does not have 'edit' rights
- the page is protected and the user does not have 'editprotected' rights (name from memory, might not be correct ☺)
- the wiki is read-only
We need to check what MediaWiki does in this case, and then assert that we do the same. (For instance, the “edit” link changes to “view source”, right?)