Soft logout

[turt2live]

see element-hq/riot-meta#239 and related issues

TODO:

• SDK: Handle new CS API error code (Soft logout: SDK: Handle new CS API error code #2584)
• SDK: Detect M_UNKNOWN_TOKEN errors not only on /sync responses -> Can be done later
• Show the login page on MXSessionStateSoftLogout without loosing user data
• Rehydrate session reusing device_id and restart
• Cache the softLogout state to display the login page directly on startup
• Implement the login UX (https://zpl.io/a89QL5x). This is likely to be an adapted version to match the current auth UI.
• Detect if all keys are backed up and use the right UX ^
• "Clear all data" button
• SSO support: Soft logout: Support SSO auth #2597