Refactor searchable? method to use_elasticsearch?

What does this MR do and why?

Related to #428070 (closed)

A smaller iteration to allow all projects to be indexed (eventually)

This MR refactors the searchable? method and replaces it with the method it calls use_elasticsearch?. This refactor reduces complexity in how a project is defined as "being indexed". As part of the refactor, I had to update a few specs.

Screenshots or screen recordings

No UI changes

How to set up and validate locally

1. Setup elasticsearch in gdk
2. Enable advanced search and elasticsearch indexing restrictions, I chose one group and project
3. Reindex everything from scratch: bundle exec rake gitlab:elastic:index
4. Verify that only the expected projects exist in the index, in my case that is 3 projects (2 under the selected group, and 1 selected project)

      curl --request GET \
     --url http://localhost:9200/gitlab-development-projects/_search \
     --data '{
   	"query": {
   		"bool": {
   			"must": [
   				{
   					"term": {
   						"type": {
   							"value": "project"
   						}
   					}		
   				}
   	    	]
   		}
   	}
   }'

5. Add a new project to the indexing restrictions setting and save
6. Verify that the new project is indexed (log/elasticsearch.log)
7. Delete a project from indexing restrictions setting and save
8. Verify that ElasticDeleteProjectWorker is called for that project (log/sidekiq.log)
9. Add a new group to the indexing restrictions and save
10. Verify that the all of the namespaces' projects are indexed (log/elasticsearch.log)
11. Delete a namespace from indexing restrictions setting and save
12. Verify that ElasticDeleteProjectWorker is called for all of the namespaces' project (log/sidekiq.log)

MR acceptance checklist

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