TASK: 9.0 remove leftover manual context path string operations

[mhsdesign]

With https://github.com/neos/neos-ui/pull/2178 we moved into a direction of rather fetching context paths from the server than manually guessing them in the client.

The current 9.0-dev still shows 21 references to code like .split('@')

const contextString = this.props.siteNodeContextPath.split('@')[1];
const subject = `${this.props.options.createNew.path}@${contextString}`;

This will probably lead to hard to discover bugs, so we should rather make sure to NEVER manipulate the contextPath in the client. (possibly even by restricting the split operation? but at least by going through the code while looking at .split('@')

[mhsdesign]

https://github.com/neos/neos-ui/blob/8404fd391cb13425e3f893014dae025d2f610dd1/packages/neos-ui-sagas/src/Browser/index.js#L5 might also be broken on 9.0

and logic such

https://github.com/neos/neos-ui/blob/02ef655858912ca12230df9104b5b5d7b79329c2/packages/neos-ui/src/Containers/ContentCanvas/index.js#L178

as well