Allow Synapse to send registration emails + choose Synapse or an external server to handle 3pid validation (#5987)

This is a combination of a few different PRs, finally all being merged into `develop`:

* #5875 
* #5876 
* #5868 (This one added the `/versions` flag but the flag itself was actually [backed out](891afb57cb (diff-e591d42d30690ffb79f63bb726200891)) in #5969. What's left is just giving /versions access to the config file, which could be useful in the future)
* #5835 
* #5969 
* #5940

Clients should not actually use the new registration functionality until https://github.com/matrix-org/synapse/pull/5972 is merged.

UPGRADE.rst, changelog entries and config file changes should all be reviewed closely before this PR is merged.

synapse/push/pusher.py

@@ -35,6 +35,7 @@ except Exception:
 class PusherFactory(object):
     def __init__(self, hs):
         self.hs = hs
+        self.config = hs.config
 
         self.pusher_types = {"http": HttpPusher}
 
@@ -42,12 +43,16 @@ class PusherFactory(object):
         if hs.config.email_enable_notifs:
             self.mailers = {}  # app_name -> Mailer
 
-            templates = load_jinja2_templates(
-                config=hs.config,
-                template_html_name=hs.config.email_notif_template_html,
-                template_text_name=hs.config.email_notif_template_text,
+            self.notif_template_html, self.notif_template_text = load_jinja2_templates(
+                self.config.email_template_dir,
+                [
+                    self.config.email_notif_template_html,
+                    self.config.email_notif_template_text,
+                ],
+                apply_format_ts_filter=True,
+                apply_mxc_to_http_filter=True,
+                public_baseurl=self.config.public_baseurl,
             )
-            self.notif_template_html, self.notif_template_text = templates
 
             self.pusher_types["email"] = self._create_email_pusher
 
@@ -78,6 +83,6 @@ class PusherFactory(object):
         if "data" in pusherdict and "brand" in pusherdict["data"]:
             app_name = pusherdict["data"]["brand"]
         else:
-            app_name = self.hs.config.email_app_name
+            app_name = self.config.email_app_name
 
         return app_name