Improve authorization to access to Timeseries data (fixes #487)

[eyobofficial]

Fixes #487

When a Timeseries group is marked as public, it is always available for viewing and downloading the Timeseries data, overwriting whatever the current value of the ENHYDRIS_OPEN_CONTENT setting value. For logged-in users, the timeseries data is only available for viewing and downloading if they are either the owner or one of the maintainers of the station.

Checklist:

• [ ] All tests are passing
• [ ] I have added any necessary documentation and/or release notes
• [ ] I have followed the commit message guidelines and the "before submitting a pull request" guidelines
• [ ] If I introduced (or abolished) any third-party library, I will write a short paragraph in this PR according to the guidelines
• [ ] I have removed all obsoleted code

[aptiko]

This has been fixed in 9cf480c021b44b2e9db01acc467e1cf0c602277b.