Voting: add contextual data fields for votes

[john-light]

It would be nice if users could easily find more contextual information about a vote.

For example, in the recent AGP-1 vote we put a link to the raw AGP-1 file directly in the vote question. This is a bit clunky. It would be nice if, instead, there were a couple of extra fields such as:

Resource link:
Discussion link:

Users could then link to a github/IPFS/swarm/ etc file for the resource link and a forum or chat room for the discussion link.

[john-light]

Related to / possibly superseded by: aragon/client#452

[sohkai]

We could make a proper spec out of the Survey metadata and support it in the Voting app: #231 (comment).

[danimesq]

This should be prioritized.

[paalex]

I can try and make a PR that makes the current input (question) multiline, to make it easier to input longer text. If I see that it's not too complicated to add additional fields, I can give it a go

[danimesq]

CC @john-light (pls remember how yearn.finance's votes have context) and consider prioritizing it.