Only use elasticsearch when it is enabled

What does this MR do?

We have two distinct settings for elasticsearch - indexing on, and search on. At present, we will sometimes search using elasticsearch even when "search on" is unset. This is because we forget to consult that setting in the search service, instead only looking at the first.

This MR solves the issue by introducing a new ::ApplicationSetting#search_using_elasticsearch?(scope:) method that encapsulates all the necessary information to make a decision. It also introduces comprehensive tests for how the settings feed into the search services.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Performance and testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team

Closes #11795 (closed)