Hxl-proxy

- update to libhxl==4.25.2 to block tagger exploit

- improve logging output

- add /api/source-info to get information about a data source (Excel-only for now)
- add support for filling merged areas in Excel to UI and data-preview API
- change references to hxl.io to hxl.input for libhxl-python 1.25+
- refactored to support new InputOptions class in libhxl-python: all input options are now available for all API calls
- all unit tests passing

- fix cache threshold type

- added warning banner to admin pages, per OICT requirements (code committed for 1.25)
- increase threshold for cache

- bugfix release: change debug handling in config for data preview (to detect non-existant sheet in an Excel workbook)
- add PyPi publish target to Makefile
- fix unit tests to work in non-debug mode