webapp: switch landing page from CI runs to conceptual benchmarks

[austin3dickey]

For a few reasons, I propose switching the landing page from the "CI runs" page to the "conceptual benchmarks" page.

1. The "CI runs" page is sometimes still slow (landing page: 'recent runs' query may still be slow occasionally (> 10 s) #1482)
2. I'd like to be freer to work on the DB schema and server query patterns without worrying as much about the performance of the landing page
3. Users don't seem to navigate to Conbench to research a specific run as often as they do to research a specific benchmark. (Unless they're linked to a run from a PR comment or something, but then they bypass the landing page anyway.)