Fixes https://github.com/vector-im/riot-web/issues/10235
CSS and copy are left as an exercise for a later iteration.
Login page handling is left for https://github.com/vector-im/riot-web/issues/10236
This implementation reuses as much of the Lifecycle flow as it can without causing problems. Most importantly, it requires https://github.com/matrix-org/matrix-js-sdk/pull/975 to be able to detect a soft logout and react to it. When it comes time to starting/stopping the Lifecycle, additional parameters are provided so that the auxiliary services can (re)start themselves without the client starting to sync.
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