fix bug with availabilities endpoint

- small improvement: enable adding phone numbers to subject columns (#519)
- small improvement: adds worker assigned (if any) to tooltip in
  appointment view (#520)
- bug fix: avoid replacing unused placeholders in template (#515)
- bug fix: subject_type was not properly retrieved from the given ID in
  the URL on the statistics view page (#516)
- bug fix: recurring events failed if no end date was provided (#517)
- bug fix: avoid negative hours in voucher duration (#502)
- typo fix: replace import with export in export data pages (#513)