Add info_type and info_text to UNIQUE constraint in xref.
Add info_type and info_text to UNIQUE constraint in xref.

at7/ensembl,james-monkeyshines/ensembl,Ensembl/ensembl,at7/ensembl,mjg17/ensembl,james-monkeyshines/ensembl,mjg17/ensembl,mjg17/ensembl,muffato/ensembl,Ensembl/ensembl,muffato/ensembl,danstaines/ensembl,james-monkeyshines/ensembl,muffato/ensembl,danstaines/ensembl,at7/ensembl,james-monkeyshines/ensembl,willmclaren/ensembl,muffato/ensembl,danstaines/ensembl,willmclaren/ensembl,Ensembl/ensembl,at7/ensembl,mjg17/ensembl,Ensembl/ensembl