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
The definition of `shellescape` says to not quote the resulting string.
http://www.ruby-doc.org/stdlib-2.1.2/libdoc/shellwords/rdoc/Shellwords.html#method-c-shellescape