Added custom emoji settings page

What does this MR do and why?

Creates the controller and route for the custom emoji settings page.

Screenshots or screen recordings

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

• Enable the custom_emoji feature flag
• Visit any merge request or issue inside of a groups projects
• Click the award emoji button
• Click the Create new emoji link

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.