Fix OR condition in public taggings

In SQL, conditions like:

```
tag_id IN (x) AND taggable_type='Debate' OR taggable_type='Proposal'
```

Don't work as intended; we need to write:

```
tag_id IN (x) AND (taggable_type='Debate' OR taggable_type='Proposal')
```

Due to this bug, we were returning taggings for proposals without
intending to do so.

Since the code was very hard to read, we're also simplifying it.

config/initializers/acts_as_taggable_on.rb

@@ -4,12 +4,10 @@ module ActsAsTaggableOn
     after_destroy :touch_taggable, :decrement_tag_custom_counter
 
     scope :public_for_api, -> do
-      where(%{taggings.tag_id in (?) and
-              (taggings.taggable_type = 'Debate' and taggings.taggable_id in (?)) or
-              (taggings.taggable_type = 'Proposal' and taggings.taggable_id in (?))},
-            Tag.where("kind IS NULL or kind = ?", "category").pluck(:id),
-            Debate.public_for_api.pluck(:id),
-            Proposal.public_for_api.pluck(:id))
+      where(
+        tag: Tag.where(kind: [nil, "category"]),
+        taggable: [Debate.public_for_api, Proposal.public_for_api]
+      )
     end
 
     def touch_taggable
@@ -35,9 +33,10 @@ module ActsAsTaggableOn
     include Graphqlable
 
     scope :public_for_api, -> do
-      where("(tags.kind IS NULL or tags.kind = ?) and tags.id in (?)",
-            "category",
-            Tagging.public_for_api.distinct.pluck("taggings.tag_id"))
+      where(
+        kind: [nil, "category"],
+        id: Tagging.public_for_api.distinct.pluck(:tag_id)
+      )
     end
 
     include PgSearch::Model

spec/models/tagging_spec.rb

@@ -0,0 +1,21 @@
+require "rails_helper"
+
+describe Tagging do
+  describe ".public_for_api" do
+    it "returns taggings for debates and proposals" do
+      create(:tag, name: "Health", kind: nil)
+      debate = create(:debate, tag_list: "Health")
+      proposal = create(:proposal, tag_list: "Health")
+
+      expect(Tagging.public_for_api.map(&:taggable)).to match_array [debate, proposal]
+    end
+
+    it "does not return taggings for other tag kinds" do
+      create(:tag, name: "Health", kind: "custom")
+      create(:debate, tag_list: "Health")
+      create(:proposal, tag_list: "Health")
+
+      expect(Tagging.public_for_api.map(&:taggable)).to be_empty
+    end
+  end
+end