Need UI to improve discoverability of bridged users

[giomfo]

See https://github.com/vector-im/vector-web/issues/1508

The conclusions were:

• Don't add a new dropdown for 'start DM' invites
• Instead, have a button to link to Scalar to manage the bridges available for locating users
• ...and include in the autocomplete options, users on the various networks. E.g. typing "trevor ribot" would offer autocomplete suggestion of @ trevor on ribot.slack.com. If the remote directory can be spidered, then it'd include spider results too.