Use Tag instead of ActsAsTaggableOn::Tag

It's shorter, it's easier to extend its behaviour, and it's easier to
integrate with other parts of our application, like translations.

config/api.yml

@@ -20,7 +20,7 @@ Debate:
     comments:           [Comment]
     public_author:      User
     votes_for:          [Vote]
-    tags:               ["ActsAsTaggableOn::Tag"]
+    tags:               [Tag]
 Proposal:
   fields:
     id:                   integer
@@ -42,7 +42,7 @@ Proposal:
     proposal_notifications: [ProposalNotification]
     public_author:        User
     votes_for:            [Vote]
-    tags:                 ["ActsAsTaggableOn::Tag"]
+    tags:                 [Tag]
 Comment:
   fields:
     id:                 integer
@@ -68,7 +68,7 @@ ProposalNotification:
     proposal_id:       integer
     public_created_at: string
     proposal:          Proposal
-ActsAsTaggableOn::Tag:
+Tag:
   fields:
     id:               integer
     name:             string