Extract shared view search/results/_issuable.html.haml

What does this MR do?

At present the merge request result page has an old design and we want it to match the issue design which was recently updated.

It seems the best way to do this was to share code so we've extracted a shared HAML template for both of these.

This MR also includes an extra preload method for Merge Requests search results as the refactor would have otherwise caused a performance regression. Now we were able to make the performance even better than it was before.

Screenshots (strongly suggested)

Before	After

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

Related to #275899 (closed)