Soft logout: Support SSO auth

[manuroe]

From riot-ios created by manuroe: element-hq/element-ios#2597

Make sure we can log in back after a soft logout on a homeserver running SSO auth.
Information to test against a SSO auth homeserver is pinned in the #softlogout-dev room.

• Check soft logout display on SSO
• If the hydrated user ID doesn't match the session's user ID, overwrite
• Pass the device_id in the fallback URL we use for SSO
• Final test when device_id is implemented on the fallback page (blocked by Soft logout: Add a device_id query parameter on login fallback page matrix-org/synapse#5755).

[bmarty]

If the hydrated user ID doesn't match the session's user ID, overwrite

For the moment, we only display a dialog to ask the user to first clear data before signing in to another account