MSC1466: Soft Logout

[erikjohnston]

Documentation: #1467

[richvdh]

this isn't implemented anywhere yet afaik

[turt2live]

the implementation is tracked here: matrix-org/synapse#4280

[turt2live]

riot-web has done an implementation at these places:

• Implement basic soft logout handling matrix-react-sdk#3177
• Require that users go to the soft logout page if they're soft logged out matrix-react-sdk#3181
• Ask for the user's password to rehydrate their soft logged out session matrix-react-sdk#3182
• Change soft logout rehydrate text if there's pending key backups matrix-react-sdk#3187
• Fix React crash when using a non-default homeserver on soft logout matrix-react-sdk#3188
• Overwrite the old session if the new creds are for a different user matrix-react-sdk#3189
• Dress up the soft logout page to look like the design matrix-react-sdk#3190
• Support SSO for rehydrating a soft-logged-out session. matrix-react-sdk#3197