Add new GraphQL type for milestones

- added the milestone type to be displayed with investments
- the corresponding spec

app/graphql/types/budget_investment_type.rb

@@ -9,5 +9,6 @@ module Types
     field :location, String, null: true
     field :comments, Types::CommentType.connection_type, null: true
     field :comments_count, Integer, null: true
+    field :milestones, Types::MilestoneType.connection_type, null: true
   end
 end

app/graphql/types/milestone_type.rb

@@ -0,0 +1,8 @@
+module Types
+  class MilestoneType < Types::BaseObject
+    field :title, String, null: true
+    field :description, String, null: true
+    field :id, ID, null: false
+    field :publication_date, GraphQL::Types::ISO8601Date, null: true
+  end
+end

app/graphql/types/proposal_type.rb

@@ -20,6 +20,7 @@ module Types
     field :title, String, null: true
     field :video_url, String, null: true
     field :votes_for, Types::VoteType.connection_type, null: true
+    field :milestones, Types::MilestoneType.connection_type, null: true
 
     def tags
       object.tags.public_for_api

app/graphql/types/query_type.rb

@@ -20,6 +20,11 @@ module Types
       argument :id, ID, required: true, default_value: false
     end
 
+    field :milestones, Types::MilestoneType.connection_type, "Returns all milestones", null: false
+    field :milestone, Types::MilestoneType, "Returns milestone for ID", null: false do
+      argument :id, ID, required: true, default_value: false
+    end
+
     field :proposals, Types::ProposalType.connection_type, "Returns all proposals", null: false
     field :proposal, Types::ProposalType, "Returns proposal for ID", null: false do
       argument :id, ID, required: true, default_value: false
@@ -84,6 +89,14 @@ module Types
       Geozone.find(id)
     end
 
+    def milestones
+      Milestone.public_for_api
+    end
+
+    def milestone(id:)
+      Milestone.find(id)
+    end
+
     def proposals
       Proposal.public_for_api
     end