Use the 3rd party location lookup API to accept third-party locations
in their native form and look up the corresponding portal room for
that location.
Also give the network dropdown some placeholder text.
Fixes https://github.com/vector-im/vector-web/issues/2374
Changes filtering on 3rd party networks to divide into portal / non portal rooms and not show portal rooms by default. Adds a special '_matrix' network for all rooms that aren't portal rooms.
Also adds ability to query 3rd party directory servers.
