Allow to create attribute values from product form

[maarcingebala]

This PR is follow-up to #1964. Just as with variant attributes, it introduces the ability to create new attribute values from the product form.

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.

[akjanik]

This block is the same as in VariantAttributeForm, maybe we can make a function from it?

[maarcingebala]

@akjanik You were right about duplicated code, so I've extracted a form mixin that handles dynamic attributes fields in the same way in both product and variant form. I think the code looks a bit better now.

Also, I've taken the opportunity and dropped get_attribute and set_attribute functions from models, that were used mostly in tests. There is definitely too many confusing utility functions related to attributes, that we should eventually organize.

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@f88d44f). Click here to learn what that means.
The diff coverage is 72.91%.

@@            Coverage Diff            @@
##             master    #1973   +/-   ##
=========================================
  Coverage          ?   84.95%           
=========================================
  Files             ?      161           
  Lines             ?     6892           
  Branches          ?      699           
=========================================
  Hits              ?     5855           
  Misses            ?      849           
  Partials          ?      188

Impacted Files	Coverage Δ
saleor/product/models.py	94.71% <ø> (ø)
saleor/data_feeds/google_merchant.py	72.32% <0%> (ø)
saleor/dashboard/product/views.py	69.31% <0%> (ø)
saleor/dashboard/product/forms.py	86.53% <87.5%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update f88d44f...7fe6045. Read the comment docs.

[mad-anne]

Shouldn't it be rather self.fields[attr.get_formfield_name()]?

[maarcingebala]

You can access fields in this way as well. It was here before so I didn't change it.