Added Hashtag entity and objects-hashtags association with auto-sync with data.tag on Object update.

priv/repo/migrations/20201221202251_create_hashtags.exs

@@ -0,0 +1,14 @@
+defmodule Pleroma.Repo.Migrations.CreateHashtags do
+  use Ecto.Migration
+
+  def change do
+    create_if_not_exists table(:hashtags) do
+      add(:name, :citext, null: false)
+      add(:data, :map, default: %{})
+
+      timestamps()
+    end
+
+    create_if_not_exists(unique_index(:hashtags, [:name]))
+  end
+end

priv/repo/migrations/20201221203824_create_hashtags_objects.exs

@@ -0,0 +1,13 @@
+defmodule Pleroma.Repo.Migrations.CreateHashtagsObjects do
+  use Ecto.Migration
+
+  def change do
+    create_if_not_exists table(:hashtags_objects) do
+      add(:hashtag_id, references(:hashtags), null: false)
+      add(:object_id, references(:objects), null: false)
+    end
+
+    create_if_not_exists(unique_index(:hashtags_objects, [:hashtag_id, :object_id]))
+    create_if_not_exists(index(:hashtags_objects, [:object_id]))
+  end
+end