This ends up being translated to ?server_name= in the matrix-js-sdk, although that has a bug at the time of writing. It converts `server_name: ['a', 'b']` to `?server_name=a,b` instead of `?server_name=a&server_name=b`
For reference: the `viaServers` option is routed through the 'join_room' action to RoomViewStore#_joinRoom which is passed directly to the js-sdk http-api#joinRoom function.
Next steps:
* Fix the js-sdk parsing
* Make the SDK generate matrix.to links with ?via=
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