Show a confirmation page during user password reset (#8004)

This PR adds a confirmation step to resetting your user password between clicking the link in your email and your password actually being reset.

This is to better align our password reset flow with the industry standard of requiring a confirmation from the user after email validation.

synapse/push/mailer.py

@@ -123,7 +123,7 @@ class Mailer:
         params = {"token": token, "client_secret": client_secret, "sid": sid}
         link = (
             self.hs.config.public_baseurl
-            + "_matrix/client/unstable/password_reset/email/submit_token?%s"
+            + "_synapse/client/password_reset/email/submit_token?%s"
             % urllib.parse.urlencode(params)
         )