Ability to customise the registration page

### Your use case

#### What would you like to do?

We can already provide custom HTML for "/welcome" and "/home" - I'd like to be able to do the same for /register

#### Why would you like to do it?

I'm hoping to use token-based registration for https://chat.ansible.im. As of right now, enabling this on my test Synapse means tokens use the fallback auth in Element Web, which looks like this:


I've highlighted the "start auth" link because it's not obvious that you need to click this. While I would clearly love full support for tokens in Element, right now just being able to customize the register page would be enough (as I have no plans to use any other forms of auth at this time). I can also make the registration page fit the rest of our branding, which is a plus.

I can image other organisations would benefit from such customization too, and it completes the set with /welcome & /home

#### How would you like to achieve it?

We are able to provide custom HTML files for "welcome" and "home" (eg https://github.com/ansible-community/ansible.im). Ideally having a "register" in this repo would work too.

### Have you considered any alternatives?

_No response_

### Additional context

_No response_

Start by locating the existing custom HTML handling for the /welcome and /home pages and compare how those routes select their files. Extend the same behavior to /register, then verify that a supplied registration HTML file is served and that the existing pages still work.

Written by the indexing model from the issue text.