DEV: Use AR enums in reviewables related code

This is the first patch of many to replace our custom enums in Ruby by
the ones provided by `ActiveRecord`.

spec/models/reviewable_history_spec.rb

@@ -14,8 +14,8 @@ RSpec.describe ReviewableHistory, type: :model do
     history = reviewable.history
     expect(history.size).to eq(3)
 
-    expect(history[0].reviewable_history_type).to eq(ReviewableHistory.types[:created])
-    expect(history[0].status).to eq(Reviewable.statuses[:pending])
+    expect(history[0]).to be_created
+    expect(history[0]).to be_pending
     expect(history[0].created_by).to eq(admin)
   end
 
@@ -26,12 +26,12 @@ RSpec.describe ReviewableHistory, type: :model do
 
     history = reviewable.history
     expect(history.size).to eq(3)
-    expect(history[1].reviewable_history_type).to eq(ReviewableHistory.types[:transitioned])
-    expect(history[1].status).to eq(Reviewable.statuses[:approved])
+    expect(history[1]).to be_transitioned
+    expect(history[1]).to be_approved
     expect(history[1].created_by).to eq(moderator)
 
-    expect(history[2].reviewable_history_type).to eq(ReviewableHistory.types[:transitioned])
-    expect(history[2].status).to eq(Reviewable.statuses[:pending])
+    expect(history[2]).to be_transitioned
+    expect(history[2]).to be_pending
     expect(history[2].created_by).to eq(admin)
   end
 
@@ -52,7 +52,7 @@ RSpec.describe ReviewableHistory, type: :model do
     history = reviewable.history
     expect(history.size).to eq(2)
 
-    expect(history[1].reviewable_history_type).to eq(ReviewableHistory.types[:edited])
+    expect(history[1]).to be_edited
     expect(history[1].created_by).to eq(moderator)
     expect(history[1].edited).to eq("category_id" => [old_category.id, nil])
   end