The get_authenticated_session() and BaseCommand.credentials_prompt()

functions both contain the same code for prompting users for their

username and password in the terminal. These need to be merged so we

don't have to maintain two copies of the same code. The functions aren't

direct replacements for one another though, they have different usages.

So instead we pull out the credentials prompt logic into a

credentials_prompt() function, which the above functions can call.
Previously, only get_authenticated_session() contained the logic for

kicking off web-based login. But we want to enable web-based login for

all rbt commands, not just the ones that call get_authenticated_session().

We now pull out the logic into an attempt_web_login() function that

anything can call when they want to authenticate a client using

web-based login.
In the next change we'll make use of this function in a new auth handler

that lets us try web-based login if enabled before falling back on Basic

auth.

Ran unit tests.
Tested using all versions of the web login, open browser, and

  debug/logging options and their .reviewboardrc config values

  on rbt login, rbt post and rbt status.
Tested passing a username, a username and password, and an API

  token to commands.
Tested passing the same auth credentials above to commands while

  also passing --web-login (or setting in in .reviewboardrc), saw

  that the auth credentials were used instead.
Tested rbt post --diff-filename=- with piping a diff into STDIN

  while also passing --web-login. Was able to login using web

  auth before the rest of the command ran.