Adding new room aliases UI is extremely confusing

### Description

Adding published room aliases UI is confusing to put it nicely.
So what happens when I try to add a new public alias?

I enter the alias (Published Addresses » Other published addresses):

Then I get a terribly misleading error:
`There was an error updating the room's alternative addresses. It may not be allowed by the server or a temporary failure occurred.`

And then I see that my new alias is added!

Well, except it isn't.

The UI right now accepts a completely different workflow, namely:
- add a **local** alias to the room (Local Addresses » Room alias)
- ...after you clicked "Show more"...
- then type the *whole*, *same* alias again (no selection list)
- then it's added without problems, and added really.

### Suggested behaviour
`Other published addresses` shall be a selection list, similar to `Main address`, which would pick one from the locally added addresses. A helpful message above the box would also inform me that I need to add a local address first.

### Version information
- **Platform**: web (in-browser)
- **Browser**: Chrome, Firefox; any version
- **URL**: private, v. 1.5.15