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

+ evaluate dynamically added datepickers (f.has_many form)
  - related issues: #469, #1593, #1651, #1813, #1862 => #1865
+ Filter form: don't pollute the URL bar with empty input values
  - related issues: #1375
+ Clear Filters button: remove return value (it's never reached)