Improve organization of Wikibase sql schema files
completed by: ☃ unicodesnowman
mentors: Katie Filbert
Wikibase sql schema files are scattered in many places across the code base, usually in repo/includes/store/sql or lib/includes/store/sql or sometimes in repo/sql and the new usage tracking table is in client/includes/Usage/Sql.
This makes it difficult for people to see what all tables there are in Wikibase and view them in gitblit or github.
We should choose a standard location for these (probably repo/sql etc.) and organize / move these schema files there. Places in the code (mostly in hooks or in repo/includes/store/sql/SqlStore.php etc) where the schema files are registered would also need to be updated.