Use the time now as a default from filter for upcoming announcements

Statistics announcements with publications are not being filtered from the present time automatically causing published statistics to appear in the 'Upcoming' filter. [Details here](https://www.pivotaltracker.com/story/show/75338234). Provides a default 'from right now' filter on upcoming statistics with (published) publications.
Would doing it like this be clearer than the `if/else`?

```ruby
from: (params.delete(:from_date) || Time.zone.now).iso8601),
```