Small UX improvements to aliases

Follow up to https://github.com/vector-im/riot-web/issues/13501

With respect to published (alt) room aliases:

- [ ] Validate published aliases so the user doesn't try to add an alias that isn't a room alias (I accidentally used @ instead of #, for example)
- [ ] Pre-empt likely failures by incorporating information about the user's permission levels and the permissions necessary to set aliases
- [ ] If (as I suspect) the change is to the entire alias list, handle failures by attempting to set it to its current value and providing feedback along the lines of "can't change / current list invalid" vs "new entry invalid"