Add markdown support for Feature Flag Descriptions

Release notes

Problem to solve

Currently, users have a small simple-text area that can be used to describe a feature flag. It's often not enough space to properly organize and describe all the pieces related to a singular feature flag.

Proposal

Upgrade the description field to match the Markdown Rich Text seen throughout the GitLab product.

Design proposal
Iteration_4

Notes:

  • The edit button has been added as a tertiary action button next to delete. Similar to an issue or MR, this should direct users to an "edit view" which allows them to edit the feature flag name and description (with full markdown).
  • How should we handle if we @mention someone in the rich text. Do we need to determine how feature flags line up with todos?