First class descriptions

[cicdw]

Summary

This PR adds formal support for Flow Group descriptions that can be edited via API. In particular, we now have the following ways of creating and managing descriptions for flows and flow groups:

• passing a description when calling mutation{create_flow} (this has been supported for a long time -- cc @jcrist in case you wanted to begin using this in Core)
• calling the set_flow_group_description mutation (this is what this PR adds)

Note that this flow group field will be treated slightly differently from other flow group fields in that the description will not be an override. Instead, it will be editable and persistent, but individual versions may still have their own descriptions displayed (but note that they won't be editable).

All descriptions will support markdown formatting when displayed.

cc: @zhen0

Importance

This will enable a more fully featured UI experience for flows, especially for those that

Checklist

This PR:

• adds new tests (if appropriate)
• adds a change file in the changes/ directory (if appropriate)