RBCommons 8 added support for the client login flow, which was released

in Review Board 5.0.5. RBCommons needed some special handling in order

to make things work with it's two-factor authentication. While working

on that, some improvements to the general client login flow code were made:

Guarding against a client url that has an invalid port.
Encoding the client name, client URL query strings.
Adding more unit tests.

This change brings those in to the Review Board client login flow. While

here I noticed the centering for the messages after success/failed client

log ins got affected from some changes to our auth forms CSS, so this

addresses that too.

Ran unit tests.
Tested the client login flow while logged in and

  logged out.