Enforce MXID grammar for new users created by appservices

[aaronraimist]

Application services like the IRC bridge are apparently allowed to register new users with uppercase letters in their usernames. matrix-org/matrix-appservice-irc#1399

Once the bridge is fixed, Synapse should start enforcing the spec.

[clokep]

In order to do this we need to mess with the logic of RegisterRestServlet.on_POST, in particular we have slightly different logic for the desired_username for appservices vs "normal" registrations:

• Appservices have a fallback to check the user property in addition to the username property. (This is fine to keep.)
• Non-appservices lowercase the desired_username.
• Non-appservices call check_username.

I think to fix this we would want to:

1. Add an extra parameter boolean parameter to check_username (is_appservice, maybe?).
2. If is_appservice, skip the check rejecting names that start with _.
3. Ensure that the lowercasing & check_username happens for all registrations.

[clokep]

Appservices have a fallback to check the user property in addition to the username property. (This is fine to keep.)

Note that this was #9547, but was removed in #15928.