cleosrv: Allow specifying why/how a forecast was resolved

Some options:

* Edit the description and add it there (requires no change, except making the description editable which needs to be done anyway #587 )
* Adding a dedicated field on the forecast. What would a good name be?
* Add a general "commenting" feature to forecasts to keep adding comments? Should this maybe supersede the estimate.reason mechanism? Probably not. Is there any other reason to add a comment to a forecast except to explain an update in the estimate or when resolving? #589

Start by reading the cleosrv forecast-resolution flow and the existing estimate.reason mechanism. Review related issues #587 and #589 for context. The issue presents several alternatives but does not select one, so completion criteria are not yet defined.

Written by the indexing model from the issue text.