Phone export and minor DataTable improvements

[krzys-h]

• DataTable state is now stored in the URL. Closes #491
• Make it possible to easily filter DataTables by status. This makes it possible to e.g. export data of only accepted participants
• Add phone number support to wwwforms
• Add phone number export support to DataTables. Closes #519. I only tested importing on https://contacts.google.com/, but it should work everywhere because the vCard format is pretty standard.

---

This change is 

[codecov[bot]]

Codecov Report

Merging #573 (adcfaa7) into master (a183c6e) will decrease coverage by 0.15%. The diff coverage is 70.83%.

@@            Coverage Diff             @@
##           master     #573      +/-   ##
==========================================
- Coverage   85.25%   85.10%   -0.16%     
==========================================
  Files          15       15              
  Lines        2327     2349      +22     
==========================================
+ Hits         1984     1999      +15     
- Misses        343      350       +7     

Impacted Files	Coverage Δ
wwwforms/forms.py	68.68% <53.84%> (-2.25%)	:arrow_down:
wwwforms/models.py	90.29% <90.90%> (-0.07%)	:arrow_down:

---

Continue to review full report at Codecov.

Legend - Click here to learn more Δ = absolute <relative> (impact), ø = not affected, ? = missing data Powered by Codecov. Last update a183c6e...adcfaa7. Read the comment docs.