Some extra proposal fields inputs only visible in the back office

[simonaszilinskas]

Is your feature request related to a problem? Please describe.
The extra fields in proposals feature is awesome but it shows the extra info in the proposal. Some information (for example the email of the proposer) would be useful to only be visible in the back office.

Describe the solution you'd like
A button in the extra field config defining the visibility of the input between:

• visible in the proposal;
• only visible for administrators in the back office;

Additional context
An increasing number of instances require this feature so we could try to co-finance it.