Summary:
Use openpyxl instead of messytables (which used xlrd internally) in import_xls.py.
Skip empty rows since excel files can easily contain huge numbers of them.
Drop support for xls files (which openpyxl doesn't support) in favour of the newer xlsx format.
Fix some details relating to python virtualenvs and dependencies, as Jenkins was failing to find new Python dependencies.
Test Plan: Mostly relying on existing tests. Updated various tests which referred to xls files instead of xlsx. Added a Python test for skipping empty rows.
Reviewers: georgegevoian
Reviewed By: georgegevoian
Differential Revision: https://phab.getgrist.com/D3406
Summary: This moves enough server material into core to run a home server. The data engine is not yet incorporated (though in manual testing it works when ported).
Test Plan: existing tests pass
Reviewers: dsagal
Reviewed By: dsagal
Differential Revision: https://phab.getgrist.com/D2552
