The final part (for now) of my work to implement a username picker in synapse itself. The idea is that we allow
`UsernameMappingProvider`s to return `localpart=None`, in which case, rather than redirecting the browser
back to the client, we redirect to a username-picker resource, which allows the user to enter a username.
We *then* complete the SSO flow (including doing the client permission checks).

The static resources for the username picker itself (in 
https://github.com/matrix-org/synapse/tree/rav/username_picker/synapse/res/username_picker)
are essentially lifted wholesale from
https://github.com/matrix-org/matrix-synapse-saml-mozilla/tree/master/matrix_synapse_saml_mozilla/res. 
As the comment says, we might want to think about making them customisable, but that can be a follow-up. 

Fixes #8876.

This was broken in #8801.

the constructor is called with a `module_api`.

Abstracts the SAML and OpenID Connect code which attempts to regenerate
the localpart of a matrix ID if it is already in use.

Checks that the localpart returned by mapping providers for SAML and
OIDC are valid before registering new users.

Extends the OIDC tests for existing users and invalid data.

Signed-off-by: Christopher Cooper <cooperc@ocf.berkeley.edu>