Convert additional templates to Jinja (#8444)

This converts a few more of our inline HTML templates to Jinja. This is somewhat part of #7280 and should make it a bit easier to customize these in the future.

synapse/config/captcha.py

@@ -28,6 +28,9 @@ class CaptchaConfig(Config):
             "recaptcha_siteverify_api",
             "https://www.recaptcha.net/recaptcha/api/siteverify",
         )
+        self.recaptcha_template = self.read_templates(
+            ["recaptcha.html"], autoescape=True
+        )[0]
 
     def generate_config_section(self, **kwargs):
         return """\