Include post author in comment pings

[trichoplax]

Fixes #2046.

In #1548 I neglected to remove 1 special case treatment of the post author, resulting in no notification when the post author is mentioned in a comment under their own post. This pull request adds a fix and tests.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.85%. Comparing base (9c10754) to head (793c54a).

Additional details and impacted files

Components	Coverage Δ
controllers	76.03% <100.00%> (-0.05%)	⬇️
helpers	85.31% <ø> (ø)
jobs	66.88% <ø> (ø)
models	93.04% <ø> (ø)
tasks	61.11% <ø> (ø)

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[trichoplax]

For prioritisation decisions, until this fix is deployed:

• A post author @ mentioned in a new thread will be notified of the new thread (but not the mention - this pull request fixes that).
• A post author @ mentioned in a comment in an existing thread will not be notified at all (neither of the new comment nor of the mention - this pull request fixes that).
• A post author not @ mentioned will be correctly notified of new threads and new comments.

[cellio]

LGTM. Tested to confirm the fixes listed in #2056 (comment) . I noticed that a post author pinged in a new comment thread now receives a notification for the pinging comment but not also the thread; this seems reasonable to me, as you're automatically being notified of the new thread indirectly and it would be too hard to say "new thread" in the notification in this case (and we don't want double notifications there).