Add CoverageChecker

[kevinrobinson]

Who is this PR for?

developers

What problem does this PR fix?

Enforces test coverage on particularly critical files. This of course is not a great measure of code quality or even test quality, but for these files we have this level of coverage now and this can help us spot gaps if they are introduced.

What does this PR do?

Fails CI if test coverage for particular files is not 100%. It can be run locally with ENABLE_RSPEC_COVERAGE_CHECKER. Also adds some test cases to authorizer.rb and fixes a typo in mentionbot.

Screenshot (if adding a client-side feature)

Checklists

Which features or pages does this PR touch?

• Travis build

Does this PR use tests to help verify we can deploy these changes quickly and confidently?

• Included specs for changes
• Improved specs for existing code in need of better test coverage
• Manual testing made more sense here

[kevinrobinson]

From Travis:

Coverage report generated for RSpec to /home/travis/build/studentinsights/studentinsights/coverage. 12901 / 13828 LOC (93.3%) covered.
CoverageChecker: On this run, checked 4 files for full coverage, all passed!
The command "ENABLE_RSPEC_COVERAGE_CHECKER=true bundle exec rspec spec" exited with 0.

[kevinrobinson]

selfie