Fix hard-coded site name in order PDFs

[NyanKiyoshi]

This replace the hard-coded Saleor eCommerce, Mirumee Software from the order PDFs stylesheet by the site name. This goes through a base PDF HTML template file containing the footer CSS code taking the data from the site settings.

Fixes #3510.

Pull Request Checklist

1. Privileged views and APIs are guarded by proper permission checks.
2. All visible strings are translated with proper context.
3. All data-formatting is locale-aware (dates, numbers, and so on).
4. Database queries are optimized and the number of queries is constant.
5. Database migration files are up to date.
6. The changes are tested.
7. The code is documented (docstrings, project documentation).
8. GraphQL schema and type definitions are up to date.
9. Changes are mentioned in the changelog.

[codecov]

Codecov Report

Merging #3526 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #3526      +/-   ##
==========================================
+ Coverage   89.87%   89.88%   +<.01%     
==========================================
  Files         241      241              
  Lines       13119    13120       +1     
  Branches     1324     1324              
==========================================
+ Hits        11791    11793       +2     
+ Misses        923      922       -1     
  Partials      405      405

Impacted Files	Coverage Δ
saleor/dashboard/order/utils.py	93.9% <100%> (+0.07%)	⬆️
saleor/order/models.py	90.43% <0%> (+0.43%)	⬆️

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[maarcingebala]

@NyanKiyoshi Please update the changelog file.