Add autocomplete search suggestions for recently viewed merge requests

What does this MR do?

This implements the same feature for merge requests that was added for issues in !40669 (merged) . We refactored a shared class RecentItems that implements the same behaviour for both. As described in !40669 (merged) the 100 most recently viewed merge request ids are stored in a redis sorted set and then when someone types in the search bar we filter the search term from those 100 MRs' titles to find matches and show the most recent 5 matching MRs.

Screenshots

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

#249091 (closed)