Add scheduled statuses

[Gargron]

Allows users to schedule a toot (with media attachments) to be published at a certain future date. This PR only includes REST API changes, no UI.

The scheduled date must be at least 5 minutes into the future. At most, 300 toots can be scheduled at the same time. Only 50 toots can be scheduled for any given day.

Fix #340

API changes

• POST /api/v1/statuses new param scheduled_at (timestamp string). When present, and more than 5 minutes into the future, the API method will return a Scheduled Status instead of Status. The Scheduled Status will be turned into a real Status roughly at the scheduled time.
• GET /api/v1/scheduled_statuses
• GET|PUT|DELETE /api/v1/scheduled_statuses/:id

Refactoring

The PR includes a refactor of the PostStatusService class to allow passing all necessary options from a single hash, which allows encoding it more easily in the scheduled_statuses table.

Overview

The high-level overview of the PR: When scheduled_at option is present, instead of creating a status, we only run status validation, and if it passes, we create an entry in scheduled_statuses which encodes the status attributes. Media attachments now also track scheduled_status_id to prevent clearing out the ones that are going to be attached later. Every 5 minutes, a scheduler iterates over the scheduled_statuses table to fetch the ones due in the next 5 minutes, and push them into a more precise Sidekiq queue. In Sidekiq, the individual statuses are created, with media attachments being unassigned from the scheduled status and assigned to the real one.

[giromide]

Apologies for this not being a code review, but this is another way to set Mastodon apart from that other service with the birds.

[hippadrone]

Will there be a limit on the number of prescheduled posts? Same question (if different answer) for media posts. Also, is there a limit for how far in the future you can schedule?

[giromide]

My suggestion was to limit scheduled posts to five, if only to match the minimum amount of time one could schedule. Five posts. Five minutes.

[tateisu]

Is there a way to list scheduled statuses and delete one?

[deutrino]

This is essential for curators. Thanks for doing this work. CRUD would be really helpful, yes.
Edit: Tumblr had a limit of 300 scheduled posts, same as the queue (they both pulled from the same limit of 300). A limit of five is WAY too low.

[joenepraat]

@Gargron Any change this will result in a UI for the web-app somewhere in the (near) future?

[tateisu]

please add check scheduled_at is after 5min or later, because the wall clock of client machine may not correct, and customized instance may have different limit.

[bajtos]

Thank you @Gargron for making this great feature happen! ❤️

Any change this will result in a UI for the web-app somewhere in the (near) future?

I would love to see this feature exposed in the web UI too 🙏

[schiessle]

Is there already a issue/feature request for the web UI to create and manage scheduled toot's I could follow?

[schiessle]

As I couldn't find a follow-up issue I created now one #10088