Allow for grouping datasets

[forman]

Server config now allows for specifying a dataset's GroupTitle and Tags.

Closes #1038

See https://github.com/xcube-dev/xcube-viewer/pull/386

Checklist:

• [x] Add unit tests and/or doctests in docstrings
• [ ] ~Add docstrings and API docs for any new/modified user-facing classes and functions~
• [x] New/modified features documented in docs/source/*
• [x] Changes documented in CHANGES.md
• [x] GitHub CI passes
• [x] AppVeyor CI passes
• [x] Test coverage remains or increases (target 100%)