Add credentials check tool

Review Request #10223 — Created Oct. 11, 2018 and discarded — Latest diff uploaded




Previously, Review Bot did not check for credentials that may have been
accidentally included in the commit. A human reviewer would have to
look out for them, but we hoped to move more of this task's burden
to Review Bot.

A new Credentials Check tool has been added to Review Bot which
looks for various key files, other sensitive files and inline embedded
AWS credentials to make sure these are not pushed to the repository.

Manual tests (correctly finds and creates issues on lines with the
credentials, or marks the first line of a file type (including
file types specified from options tab) that should not have been
included e.g. .pem files).