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 h() and shellescape()
- [x] refactor job_execution tests

### RISKS
- verify that legal messages and strings don't break due to new shell escaping code