Add and run brakeman (epic)

[hortongn]

Documentation: https://github.com/presidentbeef/brakeman

Add the brakeman gem to the Gemfile, bundle install, and then run bundle exec brakeman -q -w 2 to find any security vulnerabilities. Create a new issue for each vulnerability found.

Verify previously found Brakeman vulnerabilities:

• Brakeman: FileAccess vulnerability #147
• Brakeman: Format Validation vulnerability #148
• Brakeman: Remote Code Execution vulnerability #149
• Brakeman: Dynamic Render Path vulnerability #150

After all vulnerabilities have been found, run Brakeman automatically in CircleCI.
Example: https://github.com/uclibs/treatment_database/blob/qa/.circleci/config.yml#L86-L89

• Add brakeman and bundler-audit to CI #155

This can be multiple pull requests.