WIP: Add link to toggle dark mode

What does this MR do?

Toggle dark mode from anywhere in GitLab! Dark Mode is still in alpha, and as a result I stick to just using Indigo because I don't run into issues. Switching Dark Mode on and off would normally require digging into profile > settings > preferences in order to change it.

Thanks to @davidsawyer for helping me kick off my idea! 🚀

What's left?

• Create a new API endpoint in either rest / GraphQL
• Adding a toggle component

Screenshots

Current UX	Stretch goal: add a toggle to show state

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

The solution is still rather half baked. Right now it will toggle between Dark Mode and Indigo, as it is hard coded in javascript. Could definitely use some ❤ and refactoring to make it better.

• 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