add number to Candidates [subgraph]

[pedropregueiro]

This PR adds a number field to the ProposalCandidate graph entity.

The idea behind this change is that the current candidate ID - proposer-slug - generates really big URLs that make it a bit hard to share candidate links around or even quickly open one the same way you do with props (https:..noun-client/proposal/X)

Here are a couple of candidate URLs as we have them today:

• https://www.nouns.camp/candidates/spec-the--to-eliminate-the-put-requesting-feedback--073f0dc58e9989c827ba5b7b35570b7315652e63
• https://www.nouns.camp/candidates/placeholder-nouns--a-movie---chapter-3-4--5---7f64c6e6908de72221a06a06b4a02a6546a4d6d2

With a number index, these would become:

• https://www.nouns.camp/candidates/65
• https://www.nouns.camp/candidates/60

This can later be improved directly in the smart contract, but a subgraph fix will go a long way today.