Add ranking feature to userinfo

[falkecarlsen]

Users can click 'Rangliste' under userinfo to get their ranking in all categories:

For comparison, the SQL queries takes only ~150ms for sales during the last year.

Unfortunately this is slow as molasses, which could be alleviated by caching - but that's effort.
Currently this is all time, should probably also have a 'since Fjulefrokost' or possibly allow users to specify a timerange.

Closes #255

Suggestions and discussions from #163 and #177 should be considered for this PR

Edit: Now even more comprehensive:

[codecov-commenter]

Codecov Report

Merging #256 (00e7821) into next (92ba538) will decrease coverage by 1.07%.
The diff coverage is 25.00%.

@@            Coverage Diff             @@
##             next     #256      +/-   ##
==========================================
- Coverage   82.44%   81.37%   -1.08%     
==========================================
  Files          30       30              
  Lines        2336     2378      +42     
  Branches      167      174       +7     
==========================================
+ Hits         1926     1935       +9     
- Misses        381      413      +32     
- Partials       29       30       +1     

Impacted Files	Coverage Δ
stregsystem/urls.py	75.00% <ø> (ø)
stregsystem/views.py	54.28% <17.14%> (-6.57%)	⬇️
stregsystem/forms.py	70.96% <55.55%> (-6.31%)	⬇️

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[Zaph-x]

It is possible to apply a greater from date, than to date

[falkecarlsen]

It is possible to apply a greater from date, than to date

Fixed in a2bb5e6