symbols: use zoekt for symbol sidebar
Created by: kzh
This PR aims to improve the responsiveness of the symbol sidebar by taking advantage of the already indexed symbols data within Zoekt. This is only applicable to the HEAD branch. If there isn't an available index, it will fallback to the on demand ctags generation service.
Testing: I believe the e2e tests cover this, but I'll double check. https://github.com/sourcegraph/sourcegraph/blob/master/web/src/e2e/e2e.test.ts#L594
This closes #6078.