This adds a .reviewboardrc flag for enabling web-based login, which we leave

off by default. The reason why we leave it off by default is because we don't

currently support web-based login when authenticating RBTools on a remote

system, so it's better to let users opt-in to the feature rather than have

it on by default.
This also includes a couple of other small changes:

1. Updates the docs related to authentication.

2. Fixes a bug where we used the wrong config_key for the open browser

   option in rbt login.

3. Allows web-based login to be used when authenticating users through

   commands other than rbt login.

Tested rbt login, saw that web-based login was turned off by default and

  could use it by passing the --web option or setting WEB_LOGIN in

.reviewboardrc
Tested using rbt status without being logged in, saw that it would bring

  me to web-based login if it was enabled, and prompted for username and

  password if it wasn't enabled.