Adds a workaround to login on servers without .well_known
This MR logs in with the username part of the MXID, rather than the full MXID. It allows logging in to servers which do not provide a .well_known file.
This is a bit of a workaround, since if an MXID is @me:frog.de, but the actual hosted HS is on matrix.mystuff.xyz, the user would have to type in @me:matrix.mystuff.xyz into the MXID prompt, which is not their actual MXID. But IMO, it's better to be able to login at all, rather than not being able to login.