Store date precision with the dates on import

[bencomp]

For indexing and searching, we convert any dates other than full dates (i.e. YYYY-MM or YYYY) to full dates. We need to allow just years or year+month (done) and store the precision.

EHRI/ehri-frontend#128 will allow free text (e.g. "Spring 1943") for display, but it still needs a date to some precision. This must also be supported - display text and machine-actionable formats.

[mikesname]

Added to UI - precision is available as:

• year
• quarter
• month
• week
• day