Add JSON field for "background"

[nicebread]

We might need three fields with information for each indicator:

• "tooltip": A short mouse-over hover tooltip (targeted at users who fill in the form)
• "info": A longer text with extended information, how to fill in the form (displayed at the right side of the screen; targeted at users who fill in the form)
• "background": Background information and justification for the indicator; maybe with literature references. Not necessary for end-users who fill in the form; but relevant for administrators, or end-users who want to know more about an indicator. Display in the right column; make it collapsed by default (should not distract and only be shown upon request)

The latter might often be empty.

[alpkaanaksu]

The commit dd6f4ee148d17f1b776da9013103de37ed4a5aa1 implements this feature.

Initial state

Item with no info

Item with info

Item with info, background information expanded