Introduce support for 3rd-party auth (OAuth2 & LDAP)

Signed-off-by: Thomas Citharel <tcit@tcit.fr>

lib/service/auth/mobilizon_authenticator.ex

@@ -0,0 +1,39 @@
+defmodule Mobilizon.Service.Auth.MobilizonAuthenticator do
+  @moduledoc """
+  Authenticate Mobilizon users through database accounts
+  """
+  alias Mobilizon.Users.User
+
+  alias Mobilizon.Service.Auth.Authenticator
+
+  import Authenticator,
+    only: [fetch_user: 1]
+
+  @behaviour Authenticator
+
+  def login(email, password) do
+    require Logger
+
+    with {:user, %User{password_hash: password_hash, provider: nil} = user}
+         when not is_nil(password_hash) <-
+           {:user, fetch_user(email)},
+         {:acceptable_password, true} <-
+           {:acceptable_password, not (is_nil(password) || password == "")},
+         {:checkpw, true} <- {:checkpw, Argon2.verify_pass(password, password_hash)} do
+      {:ok, user}
+    else
+      {:user, {:error, :user_not_found}} ->
+        {:error, :user_not_found}
+
+      {:acceptable_password, false} ->
+        {:error, :bad_password}
+
+      {:checkpw, false} ->
+        {:error, :bad_password}
+    end
+  end
+
+  def can_change_email?(%User{provider: provider}), do: is_nil(provider)
+
+  def can_change_password?(%User{provider: provider}), do: is_nil(provider)
+end