Update Diff Delta calculation to negate value of trivial or purely symbolic changes

Fix pull request reviews not being synced to git provider when they were a single-comment review + test

Check if a notification is outdated before emailing it

Allow submitting a PR review without comments in repos where user isn't a contributor (thanks Mike!) + test

Allow notifications to offer multiple follow-up actions to users. Use it to allow CriticalDefects to be marked resolved from their notification.