search: Fix several things related to search

orm: Correctly label correlated query joins via sub-queries
orm: Fixup formatting of dates in queries
orm: Fixup mangling of parameter values with colons in sub-queries
misc: Fixup incorrect offset calculation in Misc::dbtime()
forms: search: Add ability to describe a field search
search: Fix crash searching by ticket due date
search: Fix wrong results if searching by a date field (unix epoch was not
    converted to MySQL date format)
search: Fix wrong results searching by date == or !=