The User#flag_level column has not been in use for a very long time. The "new" reviewable system dynamically calculates flag scores based on past performance of the user.
This PR removes flag_level from the admin user serializer (since it isn't displayed anywhere in admin user lists) and marks the column as deprecated and targeted for removal in the next minor version.
Discourse users and associated accounts are created or updated when a
user logins or connects the account using their account preferences.
This new API can be used to create associated accounts and users too,
if necessary.
Documenting the `/u/:username:/emails.json` endpoint.
Also removing some email fields from user api responses because they
aren't actually included in the response unless you are querying
yourself.
`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.
* document user endpoints, allow for empty request/response bodies
* document more user endpoints, improve debugging output if no details are specified
* document some more user endpoints
* minor cleanup
* FIX: flakey tests due to bad regex
