Extract enums

[maarcingebala]

GraphQL maintenance - this PR extracts all enums to separate enums.py files within each graphql module. Also, I've moved some scalars to their own files.

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.

[patrys]

What if we imported these at the beginning of mutations.py?

[maarcingebala]

We could do that as well. Actually, I think there is no much difference - either here or there isort will complain about the wrong order of imports. I have no idea why this happens, I spent one hour trying to debug it but found nothing.

[codecov]

Codecov Report

Merging #3523 into master will decrease coverage by 0.02%.
The diff coverage is 87.98%.

@@            Coverage Diff             @@
##           master    #3523      +/-   ##
==========================================
- Coverage   89.88%   89.86%   -0.03%     
==========================================
  Files         241      250       +9     
  Lines       13120    13157      +37     
  Branches     1324     1326       +2     
==========================================
+ Hits        11793    11823      +30     
- Misses        922      927       +5     
- Partials      405      407       +2

Impacted Files	Coverage Δ
saleor/graphql/core/types/money.py	100% <ø> (ø)	⬆️
saleor/graphql/core/types/__init__.py	100% <ø> (ø)	⬆️
saleor/graphql/discount/schema.py	92.85% <100%> (-0.25%)	⬇️
saleor/graphql/order/types.py	85.63% <100%> (-0.33%)	⬇️
saleor/graphql/product/mutations/attributes.py	93.17% <100%> (+0.03%)	⬆️
saleor/graphql/shipping/mutations.py	97.11% <100%> (+0.02%)	⬆️
saleor/graphql/checkout/types.py	86.53% <100%> (ø)	⬆️
saleor/graphql/order/schema.py	100% <100%> (ø)	⬆️
saleor/graphql/product/types.py	97.1% <100%> (+0.08%)	⬆️
saleor/graphql/product/mutations/products.py	96.05% <100%> (ø)	⬆️
... and 42 more

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