Read integration managers from account data
For https://github.com/vector-im/riot-web/issues/4913 / https://github.com/vector-im/riot-web/issues/10161
Relies on the structure defined by [MSC1957](https://github.com/matrix-org/matrix-doc/pull/1957)
This is just the bit of code to parse the user's widgets (while watching for changes) and allow for it to be the default manager.
