Dropdown bug fixes

- Added back upcoming to milestone filter

![Screen_Shot_2016-03-23_at_16.45.22](/uploads/8c31eff7e0984aad17881f6397ff17cd/Screen_Shot_2016-03-23_at_16.45.22.png)

- Fixed bug that would cause the currently selected value to disappear on filters
- Fixed bug that throw an error when filtering by upcoming when there is only a milestone in the past

Closes #14536

See merge request !3368
Signed-off-by: Rémy Coutable <remy@rymai.me>