Add picker.js, typeahead.js

- Add bower:install task to fetch bower packages then copy into
  vendor/assets
Import dateapick classic theme
Use dateapick for paid_at, billed_at

- app/views/shared/_pickadate.html.slim:
  common partial view for pickadate.
- app/assets/javascripts/pickadate_auto.js.coffee:
  JavaScript for ditto.
Remove picker.time