Use select2 dropdown to select a user on admin reports page

part of #252
Replace an old report based on new translations and edits with a new score-based report

part of #252
Add format_date_range template filter
Add paid tasks

part of #252
Add hourly_rate to User model, modify update_rates

part of #252
Adjust rendered summary report data to a new structure

Report tables structure is taken from the sketch in #252.
Summary report data is grouped by month.
Add activity chart