Disable field mapping/import for a connection

This patch adds an option to bypass the import/mapping process completely for a certain connection.
Useful if the connection is set up to a secondary RETS feed containing only the ids of all available listings.
Mapping such a connection to the same Drealty property types used by an incremental import-only connection (see #1555740: Ignore expired listings) allows this connection to be used to purge any unavailable listings without affecting fields mappings.