Switch to verification in DMs

[dbkr]

Scope of this issue is to switch verification events from going over to_device events to being sent in DM rooms. Some UI will be needed for this, eg. a prompt to create a new DM if the user doesn't already have one with both people in.

• Add a labs flag for verification in DMs.
• Support incoming verifications both in DMs and old to_device messages. If the labs flag is on, initiate verifications using DMs (interface in verification in DMs matrix-org/matrix-js-sdk#1050)
• Once other clients support verif over DMs, remove labs flag & always send over DM.
• Add tiles for verification events in rooms (nb. I think we should only show the events if we'd start a verification from them, eg. if we see verif events in non-DM rooms or for nonmatching users, just don't display anything).
• Any other UI necessary, eg. prompt to start a DM if there isn't one we can use.

• Verifications will still pop up in a dialog for now: nag bar / verif in side panel are separate tickets and can be decoupled (unless it makes sense todo them together).

[bwindels]

@dbkr What is this ticket about exactly? About using requestVerificationDM and acceptVerificationDM from the js-sdk PR in react-sdk? Or is this a meta ticket?