General performance improvements needed for stats.yml

[BryceStevenWilley]

MLH has enough snapshot rows that almost every page in InterviewStats interviews are sluggish;

• 30 seconds to load and view the snapshot rows for an interview (125k rows); surprised this even works, although the excel process doesn't work. The page itself is unusable because it's too much content for the browser to render.
• 17 seconds for the initial page to load. Fairly certain this because it's doing a count of the number of session for each interview on that loading screen, so it has to iterate over each interview individually. Could try to throw an index over the interview column of the snapshot table and see if that improves it at all.

[rajeswari1301]

Checked the index idea from the issue - filename is already indexed on both tables in prod (confirmed with pg_indexes).

Ran EXPLAIN ANALYZE on the actual queries stats.yml runs on page load:

• jsonstorage query: 1.33s, scans the whole table (expected - grouping by filename can't use the index anyway)
• One of the two session/userdict queries: 1.22s, and it's spilling to disk on around 2.5M rows
• The other session query (the one with a self-join) didn't finish before I stopped testing it on prod, but it does more work than the one above, so it's likely the heaviest of the four

Also found session_overall_stats (the one that spills to disk) is computed but never actually used anywhere in the code.

Still open: session_summary_by_filename runs on every load even though the sessions tab is hidden by default. Want to figure out the right way to defer that.