Fix CLI scheduler function

[jsit]

Fixes #1315

Proposed changes:

• Change Scheduler::schedule_post_activity to Scheduler\Post::schedule_post_activity in CLI.
• Pass post object to schedule_post_activity, not post ID

Other information:

• Have you written new tests for your changes, if applicable?

Testing instructions:

• Run wp activitypub post [delete/update] and make sure there are no errors

[pfefferle]

Thanks a lot @jsit

[jsit]

@pfefferle I think a similar thing might need to be done for the comment actions as well.