RGDParser: backport further changes from ensembl-xref

Summary:
 - new "please see the NOTICE file" copyright note
 - add more POD blocks
 - direct-xref entries now use the same source ID as xrefs themselves
 - remove the final bit of custom SQL
 - better handling of RefSeq accessions and synonym lines
 - get rid of more occurrences of postfix conditionals and "unless"
 - quite a lot of reformatting

Issue: ENSCORESW-3219