Add local database table for ESGF facets

[ScottWales]

Download the list of valid facets from ESGF to a local table.

This would allow facets to be validated for all projects, including CORDEX etc.

It would also allow use of SQL wildcards to select multiple projects, e.g. model='ACCESS%'

Requires:

• New database table esgf_facet with columns key, value
• Admin command to download facets from ESGF 'https://esgf.nci.org.au/esg-search/search/?offset=0&limit=0&facets=*' and add new entries to the database (on a cron job)
• Add validation logic to the cli and api to expand wildcards and raise an error if a facet is not valid

[paolap]

In a way we have the first "dot" not as database table but as json files.
What's still missing is the point 2 and 3

[paolap]

There is now a function in cordex.py get_esgf_facets(project) to retrieve the facets and values.
Will need to review this when cordex release is done

[paolap]

We reverted to previous behaviour just getting the facets from json file. Will have to review this later on