Zero pages design

[mad-anne]

This changes handling empty querysets in dashboard list views with is_empty flag.

Closes #1435

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

❗ No coverage uploaded for pull request base (master@fc82502). Click here to learn what that means.
The diff coverage is 100%.

@@            Coverage Diff            @@
##             master    #1513   +/-   ##
=========================================
  Coverage          ?   82.74%           
=========================================
  Files             ?      131           
  Lines             ?     5667           
  Branches          ?      642           
=========================================
  Hits              ?     4689           
  Misses            ?      813           
  Partials          ?      165

Impacted Files	Coverage Δ
saleor/dashboard/category/views.py	82.75% <100%> (ø)
saleor/dashboard/order/views.py	56.67% <100%> (ø)
saleor/dashboard/discount/views.py	66.66% <100%> (ø)
saleor/dashboard/group/views.py	100% <100%> (ø)
saleor/dashboard/staff/views.py	100% <100%> (ø)
saleor/dashboard/customer/views.py	100% <100%> (ø)
saleor/dashboard/product/views.py	70.9% <100%> (ø)
saleor/dashboard/shipping/views.py	97.22% <100%> (ø)
saleor/dashboard/sites/views.py	96.49% <100%> (ø)

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