Add before and after guardian checks

[angusmcleod]

Adds a system to allow plugins to more easily modify guardian can_ method outcomes.

Example

Currently the only way to modify guardian can_ method outcomes is via module pre-pension, for example:

module DiscourseActivityPubGuardianExtension
  def can_edit_post?(post)
    return false if post.activity_pub_remote?
    super
  end

  def can_change_post_owner?
    return false if activity_pub_enabled_topic?
    super
  end

  def activity_pub_enabled_topic?
    return false unless DiscourseActivityPub.enabled && request&.params&.[]("topic_id")
    topic = Topic.find_by(id: request.params["topic_id"].to_i)
    topic&.activity_pub_enabled
  end
end
...
Guardian.prepend DiscourseActivityPubGuardianExtension

See further

This change would mean the above could be done via the plugin api like so:

add_guardian_check(:before, :edit_post) do |guardian, result, post|
  !post.activity_pub_remote?
end

add_guardian_check(:before, :change_post_owner) do |guardian|
  return true unless DiscourseActivityPub.enabled && guardian.request&.params&.[]("topic_id")
  topic = Topic.find_by(id: guardian.request.params["topic_id"].to_i)
  !topic&.activity_pub_enabled
end

[discoursebot]

This pull request has been mentioned on Discourse Meta. There might be relevant details there:

https://meta.discourse.org/t/guardian-before-and-after-checks-api/307241/1