codeintel: Add index and indexable_repository to db layer
Created by: efritz
This PR adds a draft of the tables and additions to the db layer to support RFC 153: Automatic LSIF indexing
This PR focuses on querying/updating the lsif_indexes table and querying (but not populating) the lsif_indexable_repositories table. Finding the data to populate this table will be in a separate PR.
Note to reviewers: This PR is large mainly due to regenerated mocks.