Add toggle to issue/MR discussion sort order

What does this MR do?

Addresses the frontend first pass of #14588 (closed)

This is first pass on the frontend for sorting discussions in MR's and Issue pages under a FF. After this is merged in backend will work on adding the correct functionality to make this feature persists across users.

Repro:

• rails c
• Feature.enable(:sort_discussions)
• Go to an MR and an Issue
• Toggle between "Newest First" and "Oldest First"

Things to note:

• Adding another button to the "Show All Activity" and the "Create Merge Request" Section is making the ui glitch a little bit while fetching some resources.

• Style of the timeline feels off when we toggle to Newest First in terms of the timeline extending beyond the notes app, im not sure how UX wants to handle this.

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