Switch to using PostCreator for importing posts

[sman591]

Discourse has a PostCreator that handles a bunch of extra logic this bridge doesn't consider, and consolidates a lot of logic (such as bumping posts, #20). We should switch to using that instead of doing raw Post.create calls.

A simple example of use: https://github.com/discourse/try-bot/blob/master/plugin.rb

[sman591]

Also: https://github.com/discourse/discourse/blob/master/script/import_scripts/base.rb#L466

[sman591]

This will likely cause a feedback loop because of on(:post_created), which will need to be avoided