Fix sonar vulnerability

[zerasul]

In the last sonar report, there is a new Sonar Vulnerability.

https://sonarcloud.io/project/issues?id=blask-project-key&issues=AXMIshR2KcdFByWoFtd2&open=AXMIshR2KcdFByWoFtd2

[dukebody]

@zerasul I understand that the issue is that the user can input any filename in the URL and the system will open and render it. We need to clean it so it only allows opening files from the base directory, not traversing filepaths.

[dukebody]

The solution is to use safe_join instead of path_join I believe: https://tedboy.github.io/flask/interface_api.useful_funcs.html#flask.safe_join

[zerasul]

Thanks a lot for the help @dukebody i already merged the pull request.