This allows Review Bot's secret scanner to identify Review Board's API tokens,

which were recently updated to follow a new rbp_[A-Za-z0-9]{251} format.
This change also updates the secret scanner unit tests to ensure that any

additional validation methods for validating the secrets are called when

they're supposed to be called. Further, the unit tests were updated to

ensure that checksums are taken into account for secrets that use checksums.

Manually tested making review requests that contained new API tokens

  and saw that secret scanning picked them up.
Ran and added unit tests to

./reviewbot/tools/tests/test_rbsecretscanner.py