Relative date filters for all search forms

Allow users to enter relative date filters such as "Current year-to-date", "Current month-to-date", "Last year", etc. on all search forms. This will be particularly useful for "smart groups" (i.e. saved searches). Users will now be able to create a smart group for "people donating more than $500 this year to date" ... for example, without needing to periodically modify the smart group criteria.