authority.authority_linking needs more indexes

[Galen Charlton]

The authority.authority_linking table does not have any indexes on the source and target columns. While that table will typically have only a few tens of thousands of rows at most, adding indexes on those columns would provide a minor speedup to authority ingest and linking operations - and a major speed increase to reports that involve that table.

[Jason Boyer (jboyer)]

We've also seen in the past (action_trigger.event) that indexes are very helpful for maintaining referential integrity should the PK tables grow large.