This is the final change in a series of changes that ultimately allow

using web-based login for authentication in any rbt command.
We pass a web login callback function through to the

ReviewBoardHTTPPasswordMgr where our other auth callbacks live. We

also add a ReviewBoardWebLoginHandler auth handler which attempts

web-based login once upon the first HTTP 401. If it fails, we fall back

on our existing handlers.

Tested rbt post and rbt status passing --web-login to log in to

  a local dev server.
Tested  rbt post and rbt status passing --web-login to log in to

  the real https://reviews.reviewboard.org server.
In the above tests, made sure that the ReviewBoardWebLoginHandler

  auth handler logic was reached by rbt post, and the

get_authenticated_session logic was reached by rbt status.
Tested on RB6 and RB7.1.
Ran unit tests and add unit tests in an upcoming change.