Security Fixes for XSS and Shell Injection

These changes address cross-site scripting (XSS) in various status messages, as well as shell injection via job execution and the release tagger.

/cc @zendesk/samson

### TODO

- [x] refactor models to call sanitize and shellescape
- [x] refactor job_execution tests

### RISKS
- verify that legal messages and strings don't break due to new sanitization code
Yes, it does - good catch.  I have removed the additional quoting (shown above around job.user.name) from the exports.  So as to be clear, the other double-quotes in the code above are required to construct each string, but only job.user.name actually inserts double-quotes into the resulting string as displayed to the user.