site-admin: view potential references to feature flags

Created by: bobheadxi

Adds a search request when loading feature flags that attempts to search sourcegraph/sourcegraph on the Sourcegraph instance for (potential) mentions of each feature flag on the version of the repo that corresponds to the product build, and renders them in a collapsible view.

If no results are found, such as if the Sourcegraph instance does not have the sourcegraph/sourcegraph repository, nothing happens. One idea was to make the search request against sourcegraph.com/.api/search, but I'm not sure if reaching out to sourcegraph.com is allowed in the context of a self-hosted instance. I imagine this is more a nice-to-have than anything, and will be most useful in the context of Sourcegraph Cloud and dogfood/staging environments where the sourcegraph/sourcegraph repo will be present.

Related to https://github.com/sourcegraph/sourcegraph/issues/31182, stacked on https://github.com/sourcegraph/sourcegraph/pull/32243

Does this count as a dogfooding thing? 😛

Test plan

sg start web-standalone