Support App Links To Fast Track Sign-Ups To Custom Instances

To ease on-boarding, it would be great to empower homeserver owners to create one-click sign up links on their website. Once this link is clicked. The Element application would be opened on the signup page with the address of the custom homeserver being already pre-filled. This feature should work hand-in-hand with #26 so that if the user already has an account configured in Element, he is taken to the "add an account page" instead (and once again the homeserver address is already prefilled).

From an onboarding/conversion perspective, offering the average Joe the ability to "install an app and click on a link" vs explaining him to navigate to a page and type a server address makes a massive difference. This may look like nothing, but this makes a massive difference for regular folks, this will make the process orders of magnitudes more approachable for people.

Android seems to call this [App Links](https://developer.android.com/studio/write/app-link-indexing), however the approach described on this page seems to require mapping a specific website to an application (so with this approach homeserver owners would be required to link to element.io which is not ideal and not in the spirit of the federated and open-source nature of Matrix).

Android has support for generic links mapping a "protocol" to the default application handling the protocol (similar to mailto links opening the default email client, or even phone and map links respectively opening the default phone and maps application on a smartphone when they are clicked). I am not familiar with the way Android calls it but we would need this more generic approach here, so that homeserver owners can recommend any client they want on their website, and add a generic matrix:signup link that will be opened by the default app advertising support for that "protocol" on the user device.