DEV: Enforce dominant_color length in validation (#18309)

The `add_column` `limit` parameter has no effect on a postgres `text` column. Instead we can perform the check in ActiveRecord.

We never expect this condition to be hit - users cannot control this value. It's just a safety net.

db/migrate/20220915132547_add_dominant_color_to_uploads.rb

@@ -2,7 +2,7 @@
 
 class AddDominantColorToUploads < ActiveRecord::Migration[7.0]
   def change
-    add_column :uploads, :dominant_color, :text, limit: 6, null: true
+    add_column :uploads, :dominant_color, :text, null: true
     add_index :uploads, :id, where: "dominant_color IS NULL"
   end
 end