cleosrv: Allow specifying why/how a forecast was resolved

[omarkohl]

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