Add backend for group invitations

For #887

lib/mobilizon/invitations/invitation.ex

@@ -0,0 +1,29 @@
+defmodule Mobilizon.Invitation do
+  @moduledoc """
+  Represents a local invitation to a group.
+  """
+
+  use Ecto.Schema
+  import Ecto.Changeset
+  alias Mobilizon.Actors.Actor
+
+  schema "invitations" do
+    # We need read_after_writes because the uuid is generated by
+    # the database gen_random_uuid() function
+    field :token, :string, read_after_writes: true
+
+    # label can't be NULL in database
+    # default: "" permit to set it to "" if label is nil
+    field :label, :string, default: ""
+
+    belongs_to :group, Actor
+
+    timestamps()
+  end
+
+  def changeset(invitation, attrs) do
+    invitation
+    |> cast(attrs, [:label, :group_id])
+    |> validate_required([:group_id])
+  end
+end