Fix validation errors for timestamps in GitHub syncing to not error but ignore

[joshsmith]

Problem

Right now if there are validation errors due to timestamps being off in GitHub syncing, it appears we're seeing validating_tasks or validating_comments errors. What we should do instead is actually ensure that if the timestamps are off, that we instead say the event payload was ignored due to timestamp differences.

[begedin]

This would be a matter of interpreting a transaction outcome as an :ok.

~ 2 hours to make the changes and update tests. Bulk of the time will likely be in tests.

[joshsmith]

Related to issue raised in #1148, it looks like we may need to consider this even more fully here.

If something is syncing from GitHub but the timestamp is outdated on GitHub's side, it should not just be ignored; it likely also needs to kick off a resync to GitHub for that Task or Comment.

[joshsmith]

The passing test in #1198 can serve as the basis for writing a failing test for this.