Modernize and improve options for rbext.
Modernize and improve options for rbext.

This change makes a few improvements to `rbext` to modernize command
line parsing, help output, and console output.

`rbext --help` and each subcommand's `--help` now provides more useful
instructions, with links to useful URLs where appropriate. They're
formatted by the new `HelpFormatter` split off from `rb-site`.

`rbext --version` now works, showing the Review Board version, Python
version, and Review Board install path.

`rbext test` has new native support for `-x`, `--pdb`, and
`--with-coverage` options, and explicit documentation on how to make use
of `nosetests` options.

Console output uses the new `Console` object, which doesn't do a whole
lot in our case, but does allow us to standardize things.

Variables for manual URLs were also standardized.

Testing Done:
Checked `rbext --help`, `rbext create --help`, and `rbext test --help`
and verified the output was formatted correctly.

Ran `rbext create` and verified all the generated files were correct,
including URLs.

Ran `rbext test` with the new options and verified that they impacted
the tests I ran.

Reviewed at https://reviews.reviewboard.org/r/11426/

reviewboard/reviewboard,chipx86/reviewboard,reviewboard/reviewboard,chipx86/reviewboard,chipx86/reviewboard,reviewboard/reviewboard,chipx86/reviewboard,reviewboard/reviewboard