implement support for external custom browsers #613
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Adds support for external (non-in-app) browsers on iOS to participate in SSO-sessions and allow e.g. Citrix MDM to provide its secure browser (which in turn allows SSO for e.g. enterprise Kerberos environments).
Fixes #565 but probably raises the same questions that were left unanswered in #318
Given that this unblocks some users, it might still be valuable.