Clean up & Re-arrange the keyboard shortcuts modal

What does this MR do?

For #18140 (closed)

The MR moves some of the keyboard shortcuts around and splits shortcuts in the Issues / Merge Requests section marked MRs Only into a section just for MRs.

Why

Not just for the sake of re-factoring, I promise.

In a future MR, we'll need to add more shortcuts in another specific section - "Merge Request Commits". This should certainly live right with the other Merge Request sections. To make room in the third column, I've moved the Epics shortcuts to the first column, where they fit fairly neatly.

Splitting the MRs Only shortcuts to their own section just seemed like it was time (since there are 5 of them). It looks a lot nicer separated in my opinion.

Screenshots

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