Lock the post during replacement to ensure we have the latest version of
the tags and to ensure nobody else can modify the post until after the
replacement is finished.
Perform the replacement in a before_create callback so that it runs in a
transaction and if it fails, the transaction will rollback and the
replacement record won't be created.
Doing the replacement in a transaction isn't great because, for one
thing, it could hold the transaction open a long time, which isn't good
for the database. And two, if the transaction rolls back, the database
changes will be undone, but if the replacement file has already been saved
to disk, then it won't be undone, which could result in a dangling file.
* Move replacement tests from test/unit/upload_service_test.rb to
test/functional/post_replacement_controller_test.rb
* Move UploadService::Replacer to PostReplacementProcessor.
* Fix a minor bug where if you used the API to replace a post with a file,
the replacement would fail unless you passed an empty string for the
replacement_url.
