Store identity server in Account Data and support choosing identity server integration in User Settings

[lampholder]

Additional requirements:

• when we change IS, we'll need to fetch its policy docs and capture that the user has agreed to them
• we should probably also validate that it's really an IS
• we should also somehow validate that the HS permits access to this IS if it maintains a whitelist...

[turt2live]

Closing in favour of #5921

[lampholder]

Hey @turt2live - I think #5921 is not a dup of this, since the intention of this is to expose this as an option within UserSettings so that a user can change their IS mid flight.

It could be that that's an assumption baked into #5921, but it wasn't obvious to me.