[2.x] feat(subscriptions): add support for auto follow discussions on create

[huoxin233]

Changes proposed in this pull request:

This PR introduces a new setting in the flarum/subscriptions extension that automatically subscribes users to discussions they created.

The original followAfterReply does cover discussion creation also, but not all users will want to follow every discussion they reply to, thats why a followAfterCreate is needed.

The default preference for followAfterCreate is set to true as discussion creators usually want to know when someone replied to their discussion.

Reviewers should focus on:

Would you consider including this in the core extension? Or should it be extended using another extension, as shown in my extension: https://discuss.flarum.org/d/34680-auto-follow-discussion

There have also been quite a few discussions on this behavior over the years:

• https://discuss.flarum.org/d/38423-how-to-auto-follow-created-discussions-only-not-replied-ones
• https://discuss.flarum.org/d/38851-op-has-to-manually-sub-his-own-discussion-to-get-notification-of-replies

Personally, I would definitely like to see this in core as this is something that should come out of the box.

Screenshot

Necessity

• Has the problem that is being solved here been clearly explained?
• If applicable, have various options for solving this problem been considered?
• For core PRs, does this need to be in core, or could it be in an extension?
• Are we willing to maintain this for years / potentially forever?

Confirmed

• Frontend changes: tested on a local Flarum installation.
• Backend changes: tests are green (run composer test).
• Core developer confirmed locally this works as intended.
• Tests have been added, or are not appropriate here.

Required changes: