matrix.to parameter to prefill HS

Currently, matrix.org is always prefilled at server selection.
When I invite someone using my matrix.to link, it should have a parameter, that when opened with a logged-out Element, prefills the server name.

Use case:
Some servers provide way better performance than matrix.org

1. New user installs Element
2. Opens my matrix.to link, e.g. https://matrix.to/#/@notramo:grin.hu?recommend_hs=grin.hu
3. Server selector is prefilled with grin.hu

Start with the matrix.to link format and the logged-out Element server selector described in the issue. Define how the recommend_hs parameter is specified and handled, then verify that opening the example link pre-fills grin.hu without changing behavior for links that omit the parameter.

Written by the indexing model from the issue text.