Allow for user to specify specific instance of web client

[ch0ccyra1n]

When clicking "Continue in your browser" when selecting a web client such as Element, matrix.to should allow for the user to select a particular instance of element. For example, as a user of the instance catgirl.cloud I'd want to be able to select https://element.catgirl.cloud/ as the web client for the Continue in Browser option.

The current way using the address bar isn't exactly intuitive.

[luelista]

The current way using the address bar isn't exactly intuitive.

Additionally, as far as I understand, the address bar parameter only allows a bunch of trusted instances, and isn't persisted anyway, so you'd have to add it manually every time you encounter a matrix.to link.

How do maintainers think about adding a way to configure a custom Element URL? I'd be happy to provide a PR for that.