Does a thirdparty protocol lookup to the homeserver to get the
corresponding native/virtual user for a matrix ID. Stores the
mappings in room account data. Involves some slightly nasty workarounds
for that fact that room account data has no local echo.
Queries the homeserver for PSTN protocol support, and if found,
the add-room button on the DM rooms list section opens a context
menu instead with a 'dial pad' option as well as the current 'start chat'
dialog. Entering a number into this and pressing dial performs
a thirdparty user query for the given string and starts a DM with that
user.
