Add the ability to set per release/distribution descriptions

[pulpbot]

Author: @quba42 (quba42)

Redmine Issue: 7024, https://pulp.plan.io/issues/7024

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Right now, users can set the description at the repository level.
This description will be used in the Release file for all releases/distributions in the repository.

We could add a description field to the Release content model, that would override the repository description.

This would be a minor feature addition.

[stale]

This issue has been marked 'stale' due to lack of recent activity. If there is no further activity, the issue will be closed in another 30 days. Thank you for your contribution!

[quba42]

This issue should be subsumed into #449

In particular, see: #449 (comment)

[stale]

This issue is no longer marked for closure.