`nullable` is no longer a valid type, and types also can't be an empty
string, so just bringing a number of issues with types in compliance
with the openapi spec.
The user interface has been reorganized to show email and link invites
in the same screen. Staff has more control over creating and updating
invites. Bulk invite has also been improved with better explanations.
On the server side, many code paths for email and link invites have
been merged to avoid duplicated logic. The API returns better responses
with more appropriate HTTP status codes.
Documenting a few more endpoints so that our api docs can be
automatically generated. Made a couple other minor changes, like
including the "OK" example for our default success response.
