feat(notifications): improve deploy email subjects

[dcramer]

Improve deploy email subjects so they no longer use a reply-style Re: prefix and instead include the projects visible to the recipient.

These deploy emails were not actually threading against anything useful, and the old subject omitted the project context people use to scan notifications. This change builds the subject from the same recipient-filtered project list shown in the email body, sorts that list alphabetically, and caps longer lists with +N more to keep the subject readable.

The change is intentionally limited to the deploy email path. It also adds regression coverage for recipient-scoped deploy subjects and for group-less activity emails so future subject/threading changes do not reintroduce this behavior.

[JoshFerge]

only thought is maybe we feature flag to our orgs for a day and make sure all looks good?

[dcramer]

@JoshFerge low risk imo feature flag complexity is too high given how we use them. worst case we just revert it if for some reason it ends up being an awful idea (tho i doubt it)

[Christinarlong]

changes lgtm!

[github-actions]

Backend Test Failures

Failures on 8a53591 in this run:

tests/sentry/mail/activity/test_release.py::ReleaseTestCase::test_subject_caps_projects_at_two — log

tests/sentry/mail/activity/test_release.py:263: in test_subject_caps_projects_at_two
    assert subjects_by_recipient[self.user1.email] == self.get_prefixed_subject(
E   AssertionError: assert '[Sentry] Dep...to production' == '[Sentry] Dep...to production'
E     
E     Skipping 43 identical leading characters in diff, use -v to show
E     - h +1 more aaaaaaaaaaaa to production
E     + h +1 more to version aaaaaaaaaaaa to production
E     ?          +++++++++++

tests/sentry/mail/activity/test_release.py::ReleaseTestCase::test_simple — log

tests/sentry/mail/activity/test_release.py:175: in test_simple
    assert {msg.subject for msg in mail.outbox} == {expected_subject}
E   AssertionError: assert {'[Sentry] De...o production'} == {'[Sentry] De...o production'}
E     
E     Extra items in the left set:
E     '[Sentry] Deployed smiling-goose to version aaaaaaaaaaaa to production'
E     Extra items in the right set:
E     '[Sentry] Deployed smiling-goose aaaaaaaaaaaa to production'
E     
E     Full diff:
E       {
E     -     '[Sentry] Deployed smiling-goose aaaaaaaaaaaa to production',
E     +     '[Sentry] Deployed smiling-goose to version aaaaaaaaaaaa to production',
E     ?                                      +++++++++++
E       }

tests/sentry/mail/activity/test_release.py::ReleaseTestCase::test_subject_uses_recipient_visible_projects — log

tests/sentry/mail/activity/test_release.py:223: in test_subject_uses_recipient_visible_projects
    assert subjects_by_recipient[self.user1.email] == self.get_prefixed_subject(
E   AssertionError: assert '[Sentry] Dep...to production' == '[Sentry] Dep...to production'
E     
E     Skipping 37 identical leading characters in diff, use -v to show
E     - per-boxer aaaaaaaaaaaa to production
E     + per-boxer to version aaaaaaaaaaaa to production
E     ?          +++++++++++

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