* [ ] split config options into allowed_local_3pids and registrations_require_3pid
* [ ] simplify and comment logic for picking registration flows
* [ ] fix docstring and move check_3pid_allowed into a new util module
* [ ] use check_3pid_allowed everywhere
@erikjohnston PTAL
The 'time' caveat on the access tokens was something of a lie, since we weren't
enforcing it; more pertinently its presence stops us ever adding useful time
caveats.
Let's move in the right direction by not lying in our caveats.
Rather than reimplementing the token parsing in the various places.
This will make it easier to change the token parsing to allow access_tokens
in HTTP headers.
hs.get_handlers() can not be invoked from split out processes. Moving
the invocations down a level means that we can slowly split out
individual servlets.
* `RegistrationHandler.appservice_register` no longer issues an access token:
instead it is left for the caller to do it. (There are two of these, one in
`synapse/rest/client/v1/register.py`, which now simply calls
`AuthHandler.issue_access_token`, and the other in
`synapse/rest/client/v2_alpha/register.py`, which is covered below).
* In `synapse/rest/client/v2_alpha/register.py`, move the generation of
access_tokens into `_create_registration_details`. This means that the normal
flow no longer needs to call `AuthHandler.issue_access_token`; the
shared-secret flow can tell `RegistrationHandler.register` not to generate a
token; and the appservice flow continues to work despite the above change.
