[#3213] Partially addressed code review points.

migration rollback task changes, hashtags-related config handling tweaks, `hashtags.data` deletion (unused).

priv/repo/migrations/20201221202251_create_hashtags.exs

@@ -4,7 +4,6 @@ defmodule Pleroma.Repo.Migrations.CreateHashtags do
   def change do
     create_if_not_exists table(:hashtags) do
       add(:name, :citext, null: false)
-      add(:data, :map, default: %{})
 
       timestamps()
     end

priv/repo/migrations/20201221202252_remove_data_from_hashtags.exs

@@ -0,0 +1,15 @@
+defmodule Pleroma.Repo.Migrations.RemoveDataFromHashtags do
+  use Ecto.Migration
+
+  def up do
+    alter table(:hashtags) do
+      remove_if_exists(:data, :map)
+    end
+  end
+
+  def down do
+    alter table(:hashtags) do
+      add_if_not_exists(:data, :map, default: %{})
+    end
+  end
+end