[469 & 1375] JS cleanup, dynamic datepickers, and slim filter queries

+ evaluate dynamically added datepickers (has_many form) (thanks @saepia)
  - related issues: #469, #1593, #1651, #1813, #1862 => #1865
+ Filter form: don't pollute the URL bar with empty input values
  - related issues: #1186, #1375, #1905
+ Clear Filters button: remove return value (it's never reached)

Note that ideally these initializers would be moved into our own `app/assets/javascripts/active_admin.js.coffee` file, that way it's easier to override. But for now, let's at least have the right lines of javascript imposed on our applications.