Advanced Search should add multi project search support behind a feature flag. Multi project search will return results for Advanced Search results only. ## Proposed plan ### backend work * [x] add a feature flag (gated by user) * [x] `search_controller.rb` support multiple values for project_ids, example: `?project_ids=1,2,3` and `?project_ids=1` * [x] for backwards compatibility, we will keep `project_id` for input sent from a single select drop down UI component, the `project_id` params= may be deprecated and removed in future MRs * [x] `ee/lib/gitlab/elastic/project_search_results.rb` and `ee/lib/gitlab/elastic/search_results.rb` are the two files in play for the elasticsearch backed queries * support any current permission checks (example: Ability.allowed?) * support an array of projects vs. a singular project for each scope * code/blobs * issues * merge requests * commits * comments/notes * milestones * users - this one may be difficult as it is backed by Postgres * [ ] add `project_ids` to the logging used in the [`search_controller.rb` method `append_info_to_payload`](app/controllers/search_controller.rb#L139) ### frontend work * [ ] create an EE component to support multi select for project select * [ ] pass whether Elasticsearch is enabled to the frontend to determine which component is used Note: the new component will need to be kept in sync with the CE version, but we don't make changes to those components too often so it should be ok Once frontend work