Add metadata items to tables and databases (CaDeT)

[murdo-moj]

Added metadata items

• Audience: Internal
• Provider: Analytical Platform We don't know what provider means. The audience item tells users if they can share data or not.
• Metadata last updated
• Data last updated Temporarily hidden until the run_results are reliable
• Refresh schedule: This is based on available tags in CaDeT

Changed

• I make use now of the timestamp formatter in Django so I have changed timestamps to just be raw as datahub collects them

Questions for reviewers:

• Is it worth making Audience and Provider an Enum type? I have done this