codeintel: surface per-upload retention reasons

Created by: Strum355

Previously, it was not always easily viewable in the UI whether or why an LSIF upload is retained or not. The git object preview list only showed up to a certain number of revhashes and only for a single policy at a time. This PR surfaces the list of matching retention policies, and referencing uploads, on the LSIF upload page.

Retention policy matches (and not) list with tooltip

Retention by upload reference

Retention by upload reference tooltip

Test Plan

• Plenty of test cases for the backend in https://github.com/sourcegraph/sourcegraph/blob/nsc/upload-retention-overview/enterprise/cmd/frontend/internal/codeintel/resolvers/upload_retention_test.go
• Manually tested full flow in local dev
• Some frontend stories written for visual testing