feat(activity): add bulk activity option

[miaulalala]

• Resolves: #

Summary

TODO

• ...

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• Tests (unit, integration, api and/or acceptance) are included
• Screenshots before/after for front-end changes
• Documentation (manuals or wiki) has been updated or is not required
• Backports requested where applicable (ex: critical bugfixes)

[st3iny]

Looks good otherwise.

[nickvergessen]

But fine in general from my side

[st3iny]

@miaulalala Some test failures seem to be related:

1) Test\Activity\ManagerTest::testPublish with data set #0 (null, '')
OCP\Activity\Exceptions\IncompleteActivityException: The given event is invalid

/home/runner/actions-runner/_work/server/server/lib/private/Activity/Manager.php:113
/home/runner/actions-runner/_work/server/server/tests/lib/Activity/ManagerTest.php:244

2) Test\Activity\ManagerTest::testPublish with data set #1 ('test_author', 'test_author')
OCP\Activity\Exceptions\IncompleteActivityException: The given event is invalid

/home/runner/actions-runner/_work/server/server/lib/private/Activity/Manager.php:113
/home/runner/actions-runner/_work/server/server/tests/lib/Activity/ManagerTest.php:244

[st3iny]

Looking good now. Squash and merge? :)