Added extended dropdown to merge requests header icon

What does this MR do?

Adds a extended dropdown to the merge requests header icon that will allow for users to see both assigned and reviewer merge requests.

Query

For User#review_requested_open_merge_requests_count:

SELECT COUNT(*)
FROM "merge_requests"
INNER JOIN "projects" ON "projects"."id" = "merge_requests"."target_project_id"
LEFT JOIN project_features ON projects.id = project_features.project_id
WHERE (EXISTS (SELECT 1 FROM "project_authorizations" WHERE "project_authorizations"."user_id" = 1884221 AND (project_authorizations.project_id = projects.id)) OR projects.visibility_level IN (0,10,20))
AND ("project_features"."merge_requests_access_level" > 0 OR "project_features"."merge_requests_access_level" IS NULL)
AND ("merge_requests"."state_id" IN (1))
AND "projects"."archived" = FALSE
AND EXISTS (SELECT true FROM "merge_request_reviewers" WHERE merge_request_id = merge_requests.id AND "merge_request_reviewers"."user_id" = 1884221)

Explain results (ran on #database-lab): https://explain.depesz.com/s/GbMZ

Screenshots (strongly suggested)

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

Closes #290355 (closed)