Admin reports: Render precise values for totals in detailed reports
Record submissions for comments added from update_store command

Fix #3376
Reports: replace date with datetime in Paidtasks

- Change `date` PaidTask field to `datetime`;
- List paid tasks in Detailed Statistics.
- Show dates in ‘Registered tasks’ table in Admin/Reports and Statistics;
Reports: fix date interval when time was changed to (or from) DST
Reports: allow to add corrections as paid tasks

- Change amount field from positive integer to float;
- Add new paid task type;
- Use enum type instead of unnamed constants;
Reports: Clear effective_from field after user rates updating
Skip rqworker cache missed errors in DEBUG mode
Travis: Broadcast build failures on IRC
Cleanup: remove unused imports
Use separate cache backend for stats
Log all messages linked with stats calculation via `stats` logger

related to #3398
Use default cache if stats cache backend is not configured
Stats cache: change way how obsoletes are handled

- clear cache when making stores and directories obsolete;
- re-init cache after store or directory is resurrected;

Cache size will not grow. There will not be any inconsistency when
cache is recalculated fully (obsoletes are not recalculated).
Reports: use accurate (not rounded) values for paid task amount
Cleanup: no need for extra parameter
Fix bug introduced in fb3738f

`new_value` field is not null, save empty string instead.
Related to #3376
Editor: avoid name clash
Cleanup: readability
Cleanup: removed dead param
Editor: made messages more flexible

Not that I'm proud of this but the box needs to be adaptable to display
the close button conditionally.