Add `/` as keyboard shortcut for search

What does this MR do?

It is common in developer tools and other tools for / to focus the search bar (eg. vim, GMail, less).

Since this was not already bound in GitLab I've added this as well.

I wanted to add a test but it didn't seem the other shortcuts in this file had specific tests and since this was basically a config change I thought it may not be necessary but I'm happy to hear a suggestion about where a test could live it someone thinks it's a good idea.

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

#239195 (closed)