Measure code coverage

Running tests with `grunt coverage` reports the code coverage in the
terminal and generates an html report for better understanding what’s
left uncovered.

The syntax is the same as `grunt test`:
`grunt coverage:unit:facia` runs only the tests for `facia` package.
Abstracting part of the coverage configuration
Log disccusion ajax failures in Sentry
Add discussionId to Sentry error
Merge pull request #7021 from guardian/monitor-comments

Log disccusion ajax failures in Sentry
Remove typo
Fix silly JSHint eror
Merge pull request #7023 from guardian/typo-release-message

Remove typo
Merge pull request #6977 from guardian/coverage

Measure code coverage
Merge branch 'master' into timestamp-mobile