Filter discussion activity to view only description changes

Follow up of https://gitlab.com/gitlab-org/gitlab-ce/issues/15597, to view description changes only. So in addition to:

• Show all activity
• Show comments only
• Show history only

Also have:

• Show description updates only

This would be helpful for users who want to focus on the changing SSOT description over time.

This would be a requirements management feature as well, since it would allow users using an issue's description to represent a requirement would be able to see different versions of the requirement in history.

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