Add Search to Feature Flag User List Strategy

What does this MR do?

This allows a user with too many user lists to search for the one they wish to apply to a strategy.

For #218669 (closed)

Screenshots (strongly suggested)

Query Info

gitlabhq_development=# SELECT "operations_user_lists".* FROM "operations_user_lists" WHERE "operations_user_lists"."project_id" = 8 AND "operations_user_lists"."name" ILIKE '%test%';
gitlabhq_development=# explain (analyze, buffers) SELECT "operations_user_lists".* FROM "operations_user_lists" WHERE "operations_user_lists"."project_id" = 8 AND "operations_user_lists"."name" ILIKE '%test%';
                                                                                 QUERY PLAN                                                                                  
-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------
 Index Scan using index_operations_user_lists_on_project_id_and_name on operations_user_lists  (cost=0.14..2.17 rows=1 width=584) (actual time=0.034..0.043 rows=14 loops=1)
   Index Cond: (project_id = 8)
   Filter: ((name)::text ~~* '%test%'::text)
   Rows Removed by Filter: 4
   Buffers: shared hit=2
 Planning Time: 0.077 ms
 Execution Time: 0.071 ms
(7 rows)

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability 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
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

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