We should autocomplete mxid with the local server when inviting someone

Until we get rid of mxids it's a pain having to type the whole ID if you don't share a room with folks. We should suggest an autocomplete of the mxid with the server on which the user is registered for ease. Idem on iOS and ANdroid

The issue names no file, test, or entry point, so begin by locating the invitation flows in the iOS and Android clients. Verify the desired behavior with an account that does not share a room: entering an incomplete mxid should suggest its registered server and allow the invitation to proceed.

Written by the indexing model from the issue text.