Bridge Matrix -> Signal location

[maltee1]

Converts an m.location event to a map link in order to bridge it to signal. Let's you choose the map provider via config setting.
I ran into a bit of a problem with the different fields that provide additional information. content.body is supposed to be able to contain a description. Element just copies the content.geo_uri field there, which isn't adding anything of value, so it shouldn't be printed. content.body also contains relay sender information after the relay format is applied, so I can't exclude the field. I opted for removing the contents of geo_uri from body before prepending it, since that seemed to be the solution with the least amount of change.

This doesn't include msc3488. I'm not really sure how to account for that because I still need body in relay mode. But maybe we can worry about that when extensible events are out, since those would probably require changes to Portal.apply_relay_message_format anyway.