Search results: handle date filter values with varying precision

[kjacks]

With the ability to add date-type property filters introduced in #1803 , users can now select individual dates to filter search results by.

Though the rest of Aleph supports dates of varying precision (i.e. YYYY, YYYY-MM, or YYYY-MM-DD), the filter value processing in the backend incorrectly converts all dates to January first of the given year.

Solution 1: Modify backend date processing to support varying precision of dates

Solution 2: Utilize the date histogram for all date-type properties

[sunu]

IMO we should rather show less helpful information than wrong/misleading information. So I would prefer we used a histogram for dates until the backend can handle date precision correctly.

@pudo has made a library for precision handling and there is a PR against FtM to integrate that: alephdata/followthemoney#441. Hopefully it'll make handling date precision easier.