Add ability to follow hashtags (#18809)

db/migrate/20220714171049_create_tag_follows.rb

@@ -0,0 +1,12 @@
+class CreateTagFollows < ActiveRecord::Migration[6.1]
+  def change
+    create_table :tag_follows do |t|
+      t.belongs_to :tag, null: false, foreign_key: { on_delete: :cascade }
+      t.belongs_to :account, null: false, foreign_key: { on_delete: :cascade }, index: false
+
+      t.timestamps
+    end
+
+    add_index :tag_follows, [:account_id, :tag_id], unique: true
+  end
+end