permissions: new page visualising overall submission statistics

[GraemeWatt]

The Jupyter notebook plot_submissions_per_coordinator.ipynb makes some plots of the number of submissions from different Coordinators, including a breakdown by the different ATLAS/CMS physics groups. The Jupyter notebook plot_submissions_with_date.ipynb plots submissions per month versus time. The input data for both notebooks is generated in the form of two CSV files by running the CLI command hepdata submissions write-stats-to-files in the production environment, which in turn calls write_submissions_to_files.

The two Jupyter notebooks are updated manually from time-to-time, generally when preparing a HEPData presentation. It would be nice to have an interactive visualisation, similar to the existing "Submissions Overview" available to Admin/Coordinators, that would allow all users to see up-to-date plots of the number of submissions from different Coordinators, with options to filter by Coordinator (or categories like ATLAS/CMS/Non-LHC) and month/year.