Feature #50 - Added option to filter on date in the admin statistics page.

[Elthan]

Closes #50. Added:

• datetimepicker to jsimports in standalone-base
• form for submitting from and to dates
• from and to date when computing stats for assignments
• Code refactor for compute_stats_for_assignment with included optional from_date and to_date parameters.
• Code refactor for get_usercount_within_assignments with included optional from_date and to_date parameters.
• ID to ways to sort statistics
• More information about each assignment in the statistics page, now includes id and tags.
• Gruntfile updated to include glyphicons, angular-route, and eonasdan-bootstrap-datetimepicker
• bower.json updated to include datetimepicker
• ngRoute included in app.coffee and controllers.coffee, needed after update to angular 1.7.2
• bootstrap.less: replaced all imports with bootstrap/less/bootstrap.less import and added import for datetimepicker
• CSS for datetimepicker in statistics.less
• angular-route to base.django.html

Removed:

• Unused code which belonged to the removed statistics page

[Elthan]

Pull this after #62