[#5951] Fixed javascript error when adding milestones on field management page

Signed-off-by: Cory Johns <cjohns@slashdotmedia.com>
[#5951] Fixed AttributeError when _milestone field not of type milestone, and made behavior of all milestone-type field consistent

Signed-off-by: Cory Johns <cjohns@slashdotmedia.com>
[#5951] Display closed milestones in an optgroup

Signed-off-by: Cory Johns <cjohns@slashdotmedia.com>
[#5932] Fixed Invalid field error for removed _milestone_counts fields

Signed-off-by: Cory Johns <cjohns@slashdotmedia.com>