fix start_date filter field behaviour

[willemarcel]

We have a default start date filter set, but although it's showing on the filter form, it's only enabled if there isn't other filter field filled.

Actions:

• [ ] If OSMCha is loaded with a filter set (on the url), let the start_date field empty
• [ ] If OSMCha is loaded without filters, enable the start_date filter when the user sets any other field