Style ToastUI editor to match design mockups

This MR ensures that the ToastUI editor is styled to match the design mockups

Note: Changes in this MR are behind the richContentEditor feature flag.

To enable the feature flag:

1. Run rails c in the project
2. Run Feature.enable(:rich_content_editor) in the rails console

Test URL: http://localhost:3000/gitlab-org/gitlab-shell/-/sse/master%2FREADME.md

TODO

• Ensure the editor matches the Mockups
• Add the necesarry test coverage
• UX review
• frontend review

Screenshots

WYSIWYG mode

Markdown mode

Does this MR meet the acceptance criteria?

Conformity

• Code review guidelines
• Merge request performance guidelines
• Style guides

Epic & Design mockups

This issue is part of delivering the feature described in #214559 (closed).

Mockups: #214559 (designs)

Issue: #215121 (closed)