Remove order status field

[mad-anne]

This changes remove status field form Order and make it deduced based on related Delivery Groups statuses.

Closes #1475

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. The changes are tested.
6. The code is documented (docstrings, project documentation).
7. Python code quality checks pass: pycodestyle, pydocstyle, pylint.
8. JavaScript code quality checks pass: eslint.

[codecov-io]

Codecov Report

Merging #1481 into master will increase coverage by 0.16%.
The diff coverage is 80.55%.

@@            Coverage Diff             @@
##           master    #1481      +/-   ##
==========================================
+ Coverage   82.44%   82.61%   +0.16%     
==========================================
  Files         131      131              
  Lines        5668     5677       +9     
  Branches      647      648       +1     
==========================================
+ Hits         4673     4690      +17     
+ Misses        831      823       -8     
  Partials      164      164

Impacted Files	Coverage Δ
saleor/order/signals.py	0% <ø> (ø)	⬆️
saleor/search/backends/elasticsearch_dashboard.py	100% <ø> (ø)	⬆️
saleor/checkout/views/summary.py	44.82% <ø> (ø)	⬆️
saleor/dashboard/views.py	51.61% <0%> (ø)	⬆️
saleor/dashboard/order/views.py	56.67% <100%> (ø)	⬆️
saleor/search/documents.py	78% <100%> (ø)	⬆️
saleor/dashboard/product/views.py	70.1% <100%> (+0.07%)	⬆️
saleor/order/models.py	88.46% <100%> (+0.58%)	⬆️
saleor/core/templatetags/status.py	51.51% <100%> (ø)	⬆️
saleor/dashboard/order/forms.py	74.64% <42.85%> (+2.04%)	⬆️
... and 4 more

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