Settings: Ability to change the identity server

[manuroe]

This means:

SDK:

• Store identity server in Account Data (Ensure IS in account data is read / written as specced #2665)
• We should probably also validate that it's really an IS (TODO?)
• We should also somehow validate that the HS permits access to this IS if it maintains a whitelist... (TODO?)

App:

• Settings: Add it to the new "IDENTITY SERVER" section (https://zpl.io/brMdWo3)
• When we change IS, we'll need to fetch its policy docs and capture that the user has agreed to them
• Show a warning if the IS has no terms
• Make sure the app can run without IS

Riot-web issue: element-hq/element-web#10094