Add an order field to cotation scales

[gutard]

Fix #2475

[codecov]

Codecov Report

Merging #2498 (d54ddb9) into master (9434964) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #2498   +/-   ##
=======================================
  Coverage   97.15%   97.15%           
=======================================
  Files         265      265           
  Lines       17158    17159    +1     
=======================================
+ Hits        16669    16670    +1     
  Misses        489      489           

Impacted Files	Coverage Δ
geotrek/outdoor/admin.py	97.29% <100.00%> (ø)
geotrek/outdoor/models.py	100.00% <100.00%> (ø)

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