In rbt post, a progress bar for commit validation starts up before we

know whether the client is authenticated or not. While the progress bar

is active, we'll prompt for auth credentials. When we prompt for web

login authentication, the "Please log in to the Review Board ..." line

gets printed onto the same line as the progress bar, since the cursor is

set by the bar who is not expected to deal with any external input/output.

When we prompt for username/password authentication, the line

gets printed onto the next line because we write a new line to STDOUT

before prompting for the credentials.
This change makes the web login logs prettier by checking if a progress

bar is active and then logging a new line if so for the web-login flow.

We are using an internal attribute to check if a bar is active, since I

couldn't find any public API for this. This should be fine for our

purposes, if anything changes with that attribute then we'll just get the

ugly printing again.

Made sure things looked nice when using rbt post and other commands

(rbt login, rbt status) when passing --web-login and when using

username and password prompting.