Install app with the custom homeserver/identity server already filled in

To match the android behaviour. It'd be nice to be able to give someone a link to download the iOS app which automatically fills in the custom homeserver and identity server.

This should be possible using a similar technique to android: campaign tracking.

Compare the requested iOS behavior with the existing Android implementation, focusing on how campaign tracking carries custom homeserver and identity-server values through app installation. Identify the iOS entry point for install links and the corresponding Android flow, then verify that a generated link pre-fills both server fields after installation.

Written by the indexing model from the issue text.