Password reset email links to a style-free page rendered by the homeserver

### Steps to reproduce

Enter the forgot password flow (from login) and ask for a password reset email to be sent to you.

Click the confirmation link in the email.

### What happened?

I am sent to a plain HTML page apparently rendered by Synapse.


### What did you expect?

For the page to be styled consistent with the client experience.

### Operating system

OS X

### Browser information

Chrome

### URL for webapp

app.element.io

### Have you submitted a rageshake?

No

Trace the forgot-password flow from the login screen through the confirmation link sent by email, focusing on the plain HTML page rendered by the homeserver. Compare that page with the styling in app.element.io and identify the relevant ownership boundary; done means the reset page is styled consistently with the client experience.

Written by the indexing model from the issue text.