This cleans up the code that deals with a user attempting to login
through a different connector than they registered with. The only
functional change is that `newLoginURLFromSession` is now called
with register = false when a user has an existing account.
This commit moves the user.Manage to its own package (user/manager)
so it can import the connector package in a later commit.
For clarity, it renames "Manager" to "UserManager" using gorname.
This commit has no functional changes.
This change disables the URL fixing behavior or the router associated
with the user management schema. After this commit, URLS routing
to /api/$VERSION/users must target exactly the specified paths. In
addition, `/api/$VERSION/users/` will serve a 404
This change allows users to hit the user create endpoint, which
would previously serve a redirect rather than actually making the
associated change.
