Register / Login UX proposal

The current registration flow is heavily promoting matrix.org, which is against the goals of matrix and the server is slow.
I made a design that promotes it a little more:

https://www.figma.com/proto/b612uJLqif2ndNUmqQjIhq/Untitled?node-id=1%3A17&frame-preset-name=Desktop&scaling=min-zoom

This should be changable with a setting, since it only makes sense for app.element.io. (Other deployments are specific to a server normally, but the main instance is the one you get linked to from element.io)

Start by reviewing the registration and login flow in element-web alongside the linked Figma proposal. Determine where the current matrix.org promotion is rendered and how deployment-specific settings are handled. Done means the proposed UX is implemented and the promotion can be configured for app.element.io versus other deployments.

Written by the indexing model from the issue text.