JS cleanup, dynamic datepickers, and slim filter queries

+ evaluate dynamically added datepickers (f.has_many form)
+ Filter form: don't pollute the URL bar with empty input values
+ Clear Filters button: remove return value (it's never reached)
Merge pull request #1879 from Daxter/469-dynamic-datepickers

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