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
All these columns must never include duplicates for a single post
otherwise code breaks. All are defined in post.rb in the top in constants
but we usually prefer not to ref constants in case they change and migration
becomes inconsistent.