feat: Add a view for mentions (#3505)

- Added a new table mentions for saving user mentions
- Added a filter conversation_type in the API
- Added a view to see the mentions

db/migrate/20211201224513_create_mentions.rb

@@ -0,0 +1,13 @@
+class CreateMentions < ActiveRecord::Migration[6.1]
+  def change
+    create_table :mentions do |t|
+      t.references :user, null: false, foreign_key: true
+      t.references :conversation, null: false, foreign_key: true
+      t.references :account, index: true, null: false
+      t.datetime :mentioned_at, null: false
+      t.timestamps
+    end
+
+    add_index :mentions, [:user_id, :conversation_id], unique: true
+  end
+end