via parameters get stripped

[richvdh]

Describe the bug

via parameters are not correctly passed through to applications such as element-web

To Reproduce

1. Open a matrix.to URI with via params. For example: https://matrix.to/#/!jxlRxnrZCsjpjDubDX:matrix.org/$YAum7w2Juad2_Y5o2OjPgccZSMks8rH034uEQnG6p_w?via=matrix.org&via=envs.net&via=kaplu.mba
2. Choose to open it with element-web, giving a page like the following:
   
3. Note that the 'Continue' button omits the via params: element://vector/webapp/#/room/!jxlRxnrZCsjpjDubDX:matrix.org/$YAum7w2Juad2_Y5o2OjPgccZSMks8rH034uEQnG6p_w

Clicking on the link works ok if your server is already in the room, but otherwise produces a "no known servers" error.

Expected behavior

via params should be included in the element:// URI, so that we can join rooms not yet present on the local homeserver.

Screenshots

Desktop (please complete the following information):

• OS: ubuntu 20.04
• Browser: firefox
• Version: 92.0

[georgedorn]

Confirming this is still an issue; I ran into it over the last couple of days. It applies in particular to users on instances other than the one hosting the room. The server log shows:
[...] SynapseError: 404 - No known servers

Given this is the most obvious way to share a room via the element UI, this is a pretty bad experience for new users.