Prepare for tracker entries without priority. * Extend issues_to_quantities to count entries with priority 'None'. The regular classic template enforces a priority value, but some trackers don't, e.g. the 'fast-decomposed' variant used by Intevation (https://hg.intevation.de/roundup/fast-decomposed/) uses other fields for a scrum like workflow that show the order of issues as a float value. In order to get all issues, we also have to collect them. Implement it in a backwards compatible manner for save_stats_in_db() by a keyword argument that is off by default. This is just a preparation as the database must also accept an additional column. * Cleanup some superfluous whitespace, remove an unused import and add copyright 208 year.