This moves the deactivated user check to the method which
all login types call.

Additionally updates the application service tests to be more
realistic by removing invalid tests and fixing server names.

Fixes: #12534

Signed-off-by: Dirk Klimpel <dirk@klimpel.org>

Co-authored-by: Patrick Cloke <clokep@users.noreply.github.com>

Part of the Tchap Synapse mainlining.

This allows modules to implement extra logic to figure out whether a given 3PID can be added to the local homeserver. In the Tchap use case, this will allow a Synapse module to interface with the custom endpoint /internal_info.

This is in the context of mainlining the Tchap fork of Synapse. Currently in Tchap usernames are derived from the user's email address (extracted from the UIA results, more specifically the m.login.email.identity step).
This change also exports the check_username method from the registration handler as part of the module API, so that a module can check if the username it's trying to generate is correct and doesn't conflict with an existing one, and fallback gracefully if not.

Co-authored-by: David Robertson <davidr@element.io>

* Labeled a lot more code blocks with the appropriate type
* Fixed a couple of minor typos (missing/extraneous commas)

Signed-off-by: Sumner Evans <me@sumnerevans.com>

* Mention callbacks introduced in v1.37.0

According to the documentation introduced in https://github.com/matrix-org/synapse/pull/10062

* Mention callbacks introduced in v1.39.0

According to https://github.com/matrix-org/synapse/pull/10386 and https://github.com/matrix-org/synapse/pull/9884

* Mention callbacks introduced in v1.42.0

According to https://github.com/matrix-org/synapse/pull/10524

* Mention callbacks introduced in v1.44.0 and v1.45.0

As per https://github.com/matrix-org/synapse/pull/10898, https://github.com/matrix-org/synapse/pull/10910 and https://github.com/matrix-org/synapse/pull/10894

* Mention callbacks introduced in v1.46.0

According to https://github.com/matrix-org/synapse/pull/10548

Document Synapse's behaviour when multiple modules register the same
callback/web resource/etc.

Co-authored-by: reivilibre <oliverw@matrix.org>

Co-authored-by: Azrenbeth <7782548+Azrenbeth@users.noreply.github.com>
Co-authored-by: Brendan Abolivier <babolivier@matrix.org>