- blocked users cannot create new schemas
- users without `createpage` right (e. g. anonymous users, depending on wiki configuration) cannot create new schemas
- this should be indicated as soon as they load the special page, not only as an error when they try to submit it (part of a to-be-created subtask of T212562)
We should hopefully get the first two parts for free from MediaWiki, but should definitely add a browser test to make sure (compare T211120).
@Lydia please verify: there will be no special right/group to create schemas, right? Any normal editor can do it, just like for items?