Should adding a canonical alias to a room make it free to join?

### Description

I see (and make) the mistake of having the room invite-only despite adding an alias for people to join quite a lot. A possible solution may be to also set the room to be accessible as part of the canonical alias being added.

Some possible solutions are:
* Automatically set the room to public
* Ask inline if the user would also like to make the room public (**my preferred method**)
* Eg: "It looks like you're adding an alias. Would you like to make this room public? [yes] [no]"
* Ask with a popup

### Version information

- **Platform**: web (in-browser)
- **Browser**: Chrome 62
- **OS**: Windows 10
- **URL**: riot.im/develop

Reproduce the behavior in riot.im/develop using the reported web setup, focusing on adding a canonical room alias while the room is invite-only. First clarify which proposed behavior is wanted; done means the agreed interaction reliably handles whether the room becomes publicly joinable.

Written by the indexing model from the issue text.