Add backend for group invitations

For #887

lib/mobilizon/invitations/invitations.ex

@@ -0,0 +1,42 @@
+defmodule Mobilizon.Invitations do
+  @moduledoc """
+  The Invitations context.
+  """
+
+  alias Mobilizon.Invitation
+  alias Mobilizon.Storage.Repo
+  import Ecto.Query
+
+  def get_invitation(id), do: Repo.get(Invitation, id)
+
+  def create_invitation(attrs \\ %{}) do
+    %Invitation{}
+    |> Invitation.changeset(attrs)
+    |> Repo.insert()
+  end
+
+  def update_invitation_by_token(group_id, token, attrs) do
+    case Repo.get_by(Mobilizon.Invitation, token: token, group_id: group_id) do
+      nil ->
+        {:error, "Invitation not found"}
+
+      %Mobilizon.Invitation{} = invitation ->
+        invitation
+        |> Invitation.changeset(attrs)
+        |> Repo.update()
+    end
+  end
+
+  def delete_invitation_by_token(group_id, token) do
+    case Repo.get_by(Mobilizon.Invitation, token: token, group_id: group_id) do
+      nil -> {:error, "Invitation not found"}
+      invitation -> Repo.delete(invitation)
+    end
+  end
+
+  def list_invitations(group_id) do
+    Invitation
+    |> where([i], i.group_id == ^group_id)
+    |> Repo.all()
+  end
+end