Hide room use by verifying

Currently, when verifying someone, one gets an invitation to a room. This is unwanted, for many reasons:
- Most people will defer the invitation, as they are busy verifying - and then the verification won't proceed.
- Accepting the invitation leaves one with an unwanted room, that one needs to leave again afterwards. It is messy, both the procedure and the resulting litter.
- It is quite possible one doesn't want a private channel with the person one is verifying, so it is an invasion of privacy. The verification may be for the sake of a large group, and one might even have told Element to ignore invitations from that particular room member. In some settings (reconciliation, e.g. of a victim with a bully or abuser) it is positively unacceptable; in others (e.g. young girl with older male schoolmate) very unadvisable.
- In the case of minors or vulnerable people, parents or guardians may not want unsupervised private channels with unvetted people - especially not if their pupil and the stranger both have full power..
- Verification can even be abused to get a private channel: if the older boy can get the young girl to believe that verification makes things more safe, she'll be an easy victim.
- Often there is already a private room for the two members, and confusion will result, leading to discussions being arbitrarily distributed over two channels.
- It scares newcomers off. (Oh, this is some complicated system - first I need to verify, and now I need to accept an invitation while the verification isn't even done yet!)

Using a room under the hood is fine - that comes with the idea that everything is a room. But please don't leak that to the user by default.