Ignore trailing slashes in URL comparisons

[daniel-wer]

When opening a remote dataset, the supplied webknossos_url is compared to the active webknossos context's url. If only one of the two has a trailing slash, the comparison fails and the context's token is reset to null. This leads to permission errors when opening the remote dataset, because a token is no longer available. Instead the url comparison should succeed, regardless of any trailing slashes.

Code context: https://github.com/scalableminds/webknossos-libs/blob/ab4e538b45f41108429442c68477f7c84c23e8d9/webknossos/webknossos/dataset/dataset.py#L410

Python's rstrip method should be helpful for fixing this.

It might also make sense to scan the code for further occurrences of this issue.