fix: defer creation of auth request.

Rather than creating the auth request when the user hits /auth, pass
the arguments through to /auth/{connector} and have the auth request
created there.  This prevents a database error when using the "Select
another login method" link, and also avoids a few other error cases.

Fixes #1849, #646.

Signed-off-by: Alastair Houghton <alastair@alastairs-place.net>