Let users choose which web client to use and on which domain.

**Is your feature request related to a problem? Please describe.**
I often get to join some rooms using matrix.to links but i use a self hosted instance of element, which i open in ungoogled-chromium like a web app. For other purposes I use, for example, firefox. When I press the invite link, for instance https://matrix.to/#/#matrix.to:matrix.org, it asks which homeserver to use during preview, where I am free to select any homeserver. But even after selecting my own homeserver pressing "Continue with Element" doesn't throw me to https://element.my.homeserver. It goes to element.io, no matter the homeserver you selected to preview with. And it's not possible to select which instance of element to use.

**Describe the solution you'd like**
There are two ways to do it. First, the one I like, try to automatically detect if element (or any other web client is hosted on the server used for preview), if it fails, ask the user to specify it for themselves, or continue with element.io (or other instance of element). Second, which I don't like, although it's easier to implement, is to make it possible to select the client by yourself after pressing "Change", next to "Continue with Element".

**Describe alternatives you've considered**
Copying the link and pasting it into some chat room in the client and there pressing the link, which throws straight into the room.

**Additional context**
Please, let's not make matrix that centered around vector.im, matrix.org, element.io and matrix.to, please.
Edit: Also, your feature request form is pretty darn good

Start at the matrix.to invite-link preview flow, focusing on the "Change" and "Continue with Element" actions described in the issue. Define how client and homeserver choices should be detected or selected, then verify that an invite link opens the chosen web client on the selected domain, with a fallback to element.io.

Written by the indexing model from the issue text.