ProposedIntent to link Intent and Proposal

[mayel]

Follows from #15 for more fleshed out Intents

Represents many-to-many relationships between Proposals and Intents, supporting
including intents in multiple proposals, as well as a proposal including
multiple intents.

type ProposedIntent {
  id: ID!

  """
  This is a reciprocal intent of this proposal, not primary. Not meant to be used for intent matching.
  """
  reciprocal: Boolean

  """The intent which is part of this published proposal."""
  publishes: Intent!

  """The published proposal which this intent is part of."""
  publishedIn: Proposal!
}

type ProposedIntentResponse {
  proposedIntent: ProposedIntent
}