Respect Rails parameter "_destroy" when set to 1
Fix edit state for ingest form and associations

Adds setting of persisted on associations by translator so that
removal of an element can be handled properly - i.e., removing an item
which has been persisted is different than removing an item dynamically
added after the initial item load.
Fix API error

Removes second parameter to AttributesToForm.from - it was
inconsistent and unused
Implement _destroy in a less naive way

Flags items for destruction rather than simply ignoring them.  This is
explained in the comment on `Metadata::Ingest::Association#destroy!`,
but is necessary for updating items via a translator that otherwise
wouldn't know about the existing records needing to be removed.
Handle destroyed associations in translator

Properly sets attribute to nil when all form data mapped to attribute
are marked for destruction
Merge branch 'feature/respect-the-destroy'
Bump version to 2.1.0