This PR adds a new category setting which is a column in the `categories` table, `allow_unlimited_owner_edits_on_first_post`.
What this does is:
* Inside the `can_edit_post?` method of `PostGuardian`, if the current user editing a post is the owner of the post, it is the first post, and the topic's category has `allow_unlimited_owner_edits_on_first_post`, then we bypass the check for `LimitedEdit#edit_time_limit_expired?` on that post.
* Also, similar to wiki topics, in `PostActionNotifier#after_create_post_revision` we send a notification to all users watching a topic when the OP is edited in a topic with the category setting `allow_unlimited_owner_edits_on_first_post` enabled.
This is useful for forums where there is a Marketplace or similar category, where topics are created and then updated indefinitely by the OP rather than the OP making new topics or additional replies. In a way this acts similar to a wiki that only one person can edit.
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