Ensure email validation link parameters are URL-encoded (#6063)

The validation links sent via email had their query parameters inserted without any URL-encoding. Surprisingly this didn't seem to cause any issues, but if a user were to put a `/` in their client_secret it could lead to problems.

changelog.d/6063.bugfix

@@ -0,0 +1 @@
+Ensure query parameters in email validation links are URL-encoded.

synapse/push/mailer.py

@@ -136,10 +136,11 @@ class Mailer(object):
                 group together multiple email sending attempts
             sid (str): The generated session ID
         """
+        params = {"token": token, "client_secret": client_secret, "sid": sid}
         link = (
             self.hs.config.public_baseurl
-            + "_matrix/client/unstable/password_reset/email/submit_token"
-            "?token=%s&client_secret=%s&sid=%s" % (token, client_secret, sid)
+            + "_matrix/client/unstable/password_reset/email/submit_token?%s"
+            % urllib.parse.urlencode(params)
         )
 
         template_vars = {"link": link}
@@ -163,10 +164,11 @@ class Mailer(object):
                 group together multiple email sending attempts
             sid (str): The generated session ID
         """
+        params = {"token": token, "client_secret": client_secret, "sid": sid}
         link = (
             self.hs.config.public_baseurl
-            + "_matrix/client/unstable/registration/email/submit_token"
-            "?token=%s&client_secret=%s&sid=%s" % (token, client_secret, sid)
+            + "_matrix/client/unstable/registration/email/submit_token?%s"
+            % urllib.parse.urlencode(params)
         )
 
         template_vars = {"link": link}