Ensure that jinja loader is not registered at each render

The env.loader was replaced by a new ChoiceLoader wrapping the previous
loader on every render. It seems that the `_loaded` attribute was here
to avoid re-registering the loader but it was set to `True` on the
handler instance and not on its class making it useless since the a
handler instance is created for each request.

Closes #114.

nativeauthenticator/handlers.py

@@ -14,19 +14,17 @@
 
 
 class LocalBase(BaseHandler):
+    _template_dir_registered = False
+
     def __init__(self, *args, **kwargs):
-        self._loaded = False
         super().__init__(*args, **kwargs)
-
-    def _register_template_path(self):
-        if self._loaded:
-            return
-        self.log.debug('Adding %s to template path', TEMPLATE_DIR)
-        loader = FileSystemLoader([TEMPLATE_DIR])
-        env = self.settings['jinja2_env']
-        previous_loader = env.loader
-        env.loader = ChoiceLoader([previous_loader, loader])
-        self._loaded = True
+        if not LocalBase._template_dir_registered:
+            self.log.debug('Adding %s to template path', TEMPLATE_DIR)
+            loader = FileSystemLoader([TEMPLATE_DIR])
+            env = self.settings['jinja2_env']
+            previous_loader = env.loader
+            env.loader = ChoiceLoader([previous_loader, loader])
+            LocalBase._template_dir_registered = True
 
 
 class SignUpHandler(LocalBase):
@@ -35,7 +33,6 @@ async def get(self):
         if not self.authenticator.enable_signup:
             raise web.HTTPError(404)
 
-        self._register_template_path()
         html = self.render_template(
             'signup.html',
             ask_email=self.authenticator.ask_email_on_signup,
@@ -112,7 +109,6 @@ class AuthorizationHandler(LocalBase):
     """Render the sign in page."""
     @admin_only
     async def get(self):
-        self._register_template_path()
         html = self.render_template(
             'autorization-area.html',
             ask_email=self.authenticator.ask_email_on_signup,
@@ -133,7 +129,6 @@ class ChangePasswordHandler(LocalBase):
 
     @web.authenticated
     async def get(self):
-        self._register_template_path()
         html = self.render_template('change-password.html')
         self.finish(html)
 
@@ -153,7 +148,6 @@ async def post(self):
 class LoginHandler(LoginHandler, LocalBase):
 
     def _render(self, login_error=None, username=None):
-        self._register_template_path()
         return self.render_template(
             'native-login.html',
             next=url_escape(self.get_argument('next', default='')),