Select reports by month, use previous/next month navigation

Fix #305, #306
Fix flot stack bars rendering

Don't draw zero height bar
Fix max_day_score calculating for daily chart in reports
Cleanup: Use `content` block for setting a content for a report
Show current month report if there is no month parameter
Make detailed reports template working in Django detail view

Rename context attribute user => object
Cleanup: remove unused context parameter
Create separate functions for getting of context
Use an absolute URL for filling detailed report href
Split a report template
Create an user report based on admin user reports

Fix #313