emails: Use common_context for email change notifications.

This also lets us remove `realm_uri`.

templates/zerver/emails/confirm_new_email.source.html

@@ -10,7 +10,7 @@
 
 <p>
     We received a request to change the email address for the Zulip
-    account on {{ realm.uri }} from {{ old_email }} to {{ new_email }}.
+    account on {{ realm_uri }} from {{ old_email }} to {{ new_email }}.
     If you would like to confirm this change, please click here:
     <a class="button" href="{{ activate_url }}">Confirm email change</a>
 </p>

templates/zerver/emails/confirm_new_email.txt

@@ -1,7 +1,7 @@
 Hi!
 
 We received a request to change the email address for the Zulip account on
-{{ realm.uri }} from {{ old_email }} to {{ new_email }}. If you would like
+{{ realm_uri }} from {{ old_email }} to {{ new_email }}. If you would like
 to confirm this change, please click this link:
 
 {{ activate_url }}

zerver/lib/actions.py

@@ -649,8 +649,13 @@ def do_start_email_change_process(user_profile, new_email):
                                            user_profile=user_profile, realm=user_profile.realm)
 
     activation_url = create_confirmation_link(obj, user_profile.realm.host, Confirmation.EMAIL_CHANGE)
-    context = {'realm': user_profile.realm, 'old_email': old_email, 'new_email': new_email,
-               'activate_url': activation_url}
+    from zerver.context_processors import common_context
+    context = common_context(user_profile)
+    context.update({
+        'old_email': old_email,
+        'new_email': new_email,
+        'activate_url': activation_url
+    })
     send_email('zerver/emails/confirm_new_email', to_email=new_email,
                from_name='Zulip Account Security', from_address=FromAddress.NOREPLY,
                context=context)