Restricting ContributionReward proposals

From Telegram chat. I think this would be useful:

Eric Arsenault:
is it possible to create a DAO which can only ask 1 proposal? (pre selected)... What I'm thinking of looks like the Contribution Reward, but instead of having fields you can edit when submitting a proposal, DAO participants would only be able to select from a few pre-populated proposals

Jordan Ellis:
I see, so for instance you could restrict one scheme to only ask for small amounts of reward? This would require a modification to the current ContributionReward scheme, but I think it's a great feature addition :D

Start by locating the current ContributionReward scheme and reviewing how its proposal fields are submitted. Clarify which pre-populated proposals should be allowed and how participants select them; done means the scheme can restrict participants to those choices instead of editable fields.

Written by the indexing model from the issue text.