Add information on comprehensiveness of data for each donor

[riceissa]

This is similar to the fields donations_data_update_regularity, donations_data_dlw_update_regularity, donations_data_update_lag, and donations_data_dlw_update_lag. But rather than about the regularity or lag, it would be about how comprehensive the data on Donations List Website is for each donor.

Some ideas:

• For many donors, this can be something like "all public donations" or "all public donations we know about".
• For donors like BMGF or Hewlett, this can be "all donations listed in IATI" or "all international donations" or something along those lines. The IATI registry has related metadata, e.g. "Organisations / agencies covered", "Record Exclusions", and "Thresholds". Examples:
  • https://iatiregistry.org/publisher/about/bmgf
  • https://iatiregistry.org/publisher/about/hewlett-foundation
  • https://iatiregistry.org/publisher/about/dfid
• If there is a good way to know the total donations and the donations we have recorded, we can calculate a percentage along various metric (e.g. % number of donations, % dollar amount).