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.