Improve default invitation template

[maxidorius]

There is no mechanism in the spec to automate a registration process with a pending 3PID invite. Therefore, we should make the default templates related to 3PID room invites to be more user-friendly and explain what should be done (register with the relevant 3PID).

Also talk about possible registration being denied if no or unknown 3PID is given, if the homeserver is integrated with the upcoming Registration control feature of mxisd