implement proposal type petition

[brenzi]

closes #383

[clangenb]

LGTM. Quite simple. 🥳

[clangenb]

Hmm, weird fmt.

[pifragile]

LGTM 👍
One thing to keep in mind: We only allow one concurrent proposal per ProposalActionIdentifier. This means one Petition per community and one global one. So if there are multiple petitions being voted on in one community, as soon as one gets enacted, the others will be cancelled. While this implementation makes sense for things like updating the demurrage, for petitions this would be debatable.

[brenzi]

Good point. We don't want to cancel petitions. Will look into it

edit:

• there may be other proposal types which should not supersede the same type, like treasury spends. So I introduced a helper fn to define the behavior per action type

[pifragile]

Good point. We don't want to cancel petitions. Will look into it

edit:

• there may be other proposal types which should not supersede the same type, like treasury spends. So I introduced a helper fn to define the behavior per action type

seems legit!