Allow queries by delegator and delegate for a single delegation

[oliverbarnes]

Right now single delegations are only queriable by id.

[oliverbarnes]

Explaining in more detail, what's meant here is being able to do queries like these:

query {
  delegations(delegatorEmail: ..., delegateEmail: ...) {
    ...
  }    
}

and

query {
  delegations(delegatorId: ..., delegateId: ...) {
    ...
  }    
}

[jinjagit]

TLDR: a particular delegator/delegate pair is not (will not be) sufficient to identify a unique delegation.

This query would return a list of matching delegations (or an empty list), since we are aiming for being able to make multiple different proposal-specific delegations which could be for same delegator-delegate pair. Hence, this query will return multiple delegations if multiple matches exist.

@oliverbarnes Are we OK with this type of query returning a list, or am I missing something here?