Add alt text to Product thumbnail and background_image of Collection and Category

[fowczarek]

I want to merge this change because resolve #3257

To do:

• Add alt' text filed in the Image` type
• Add background_image_alt to Collection model
• Expose background_image_alt to Collection in API
• Handle background_image_alt of Collection in dashboard
• Handle background_image_alt of Collection in dashboard2.0
• Add background_image_alt to Category model
• Expose background_image_alt to Category in API
• Handle background_image_alt of Category in dashboard
• Handle background_image_alt of Category in dashboard2.0
• Update thumbnail_url field to use ProductImage type

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.

[codecov]

Codecov Report

Merging #3429 into master will increase coverage by <.01%.
The diff coverage is 90.19%.

@@            Coverage Diff             @@
##           master    #3429      +/-   ##
==========================================
+ Coverage   89.73%   89.74%   +<.01%     
==========================================
  Files         237      237              
  Lines       12912    12925      +13     
  Branches     1308     1304       -4     
==========================================
+ Hits        11587    11599      +12     
  Misses        921      921              
- Partials      404      405       +1

Impacted Files	Coverage Δ
saleor/dashboard/category/forms.py	100% <ø> (ø)	⬆️
saleor/dashboard/collection/forms.py	100% <ø> (ø)	⬆️
saleor/checkout/views/__init__.py	83.58% <0%> (-1.27%)	⬇️
saleor/graphql/product/mutations/products.py	96.25% <100%> (+0.01%)	⬆️
saleor/product/templatetags/product_images.py	96.92% <100%> (+0.14%)	⬆️
saleor/graphql/order/types.py	85.95% <100%> (ø)	⬆️
saleor/seo/schema/email.py	100% <100%> (ø)	⬆️
saleor/product/models.py	93.02% <100%> (+0.66%)	⬆️
saleor/data_feeds/google_merchant.py	71.92% <33.33%> (-0.64%)	⬇️
saleor/graphql/product/types.py	97.3% <96.55%> (+0.49%)	⬆️
... and 3 more

---

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[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@3e6799e). Click here to learn what that means.
The diff coverage is 89.09%.

@@            Coverage Diff            @@
##             master    #3429   +/-   ##
=========================================
  Coverage          ?   89.76%           
=========================================
  Files             ?      237           
  Lines             ?    12938           
  Branches          ?     1312           
=========================================
  Hits              ?    11614           
  Misses            ?      920           
  Partials          ?      404

Impacted Files	Coverage Δ
saleor/dashboard/category/forms.py	100% <ø> (ø)
saleor/dashboard/collection/forms.py	100% <ø> (ø)
saleor/checkout/views/__init__.py	83.58% <0%> (ø)
saleor/product/templatetags/product_images.py	96.92% <100%> (ø)
saleor/graphql/order/types.py	85.95% <100%> (ø)
saleor/seo/schema/email.py	100% <100%> (ø)
saleor/product/models.py	93.02% <100%> (ø)
saleor/data_feeds/google_merchant.py	71.92% <33.33%> (ø)
saleor/graphql/product/mutations/products.py	96.03% <83.33%> (ø)
saleor/graphql/product/types.py	97.3% <96.55%> (ø)

---

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Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 3e6799e...b5279fc. Read the comment docs.