Refactors script to follow conventions of other importers and adds some features including like import, processing of post raw text, and, if needed, SSO import.
This reduces chances of errors where consumers of strings mutate inputs
and reduces memory usage of the app.
Test suite passes now, but there may be some stuff left, so we will run
a few sites on a branch prior to merging
This changes the content of `@categories_lookup` from `Category` objects
to IDs since the category names aren't needed anymore. The lookup
method has been renamed too.
