Add `pleroma_internal` as an internal field

This is a tiny step towards #1752, as listing pleroma_internal within Constants will at least cause it to be stripped by the Transmogrifier. I'll be making use of the field in the Mastodon->Pleroma migrator #162 (closed)

I agreed with your suggestions to call it "pleroma-internal", but after seeing the other field names I thought an underscore made more sense. Even better that it can be an atom without quotes.

P.S. I hope when the other fields are moved into pleroma_internal they can be merged into it if it already exists, so the custom values from my script can be preserved.