Add admin analytics initial charts (search, code-intel, users, notebooks) mvp. part 1.

Created by: erzhtor

Part 1 of https://github.com/sourcegraph/sourcegraph/issues/37686.

Description

This is MVP of the admin analytics project.

NOTE: We are not changing any data storing architecture and using only the existing 3 months of event logs data. We will be adding an event aggregation layer in the August 22 release.

This PR:

• Adds "analytics / searches" analytics chart loom video
• Adds "analytics / code-inte" analytics chart
• Adds "analytics / users" analytics chart
• Adds "analytics / notebooks" analytics chart

TODO:

• Add user activity by period @erzhtor
• Add search/precise/cross-repo percentage based on code intel hover events @thenamankumar
• Add new events to track search/precise/cross-repo results on click
• Add detailed search stats

Test plan

• sg start
• Sign in as an admin user
• Force enabled admin analytics feature flag https://sourcegraph.test:3443/search?feature-flag-key=admin-analytics-enabled&feature-flag-value=true
• Go to https://sourcegraph.test:3443/site-admin/analytics/search

Screenshot