Rename items to lines

[akjanik]

I want to merge this change because...

Fixes #1388

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 #1414 into master will increase coverage by 0.23%.
The diff coverage is 80%.

@@            Coverage Diff             @@
##           master    #1414      +/-   ##
==========================================
+ Coverage    82.9%   83.14%   +0.23%     
==========================================
  Files         128      129       +1     
  Lines        5775     5855      +80     
  Branches      663      665       +2     
==========================================
+ Hits         4788     4868      +80     
- Misses        830      831       +1     
+ Partials      157      156       -1

Impacted Files	Coverage Δ
saleor/dashboard/order/utils.py	100% <ø> (ø)	⬆️
saleor/userprofile/views.py	36.11% <ø> (ø)	⬆️
saleor/dashboard/order/views.py	56.95% <ø> (ø)	⬆️
saleor/dashboard/views.py	53.12% <ø> (ø)	⬆️
saleor/dashboard/order/forms.py	73.39% <0%> (ø)	⬆️
saleor/order/models.py	88.62% <100%> (-0.4%)	⬇️
saleor/dashboard/customer/filters.py	100% <0%> (ø)	⬆️
saleor/dashboard/shipping/filters.py	100% <0%> (ø)	⬆️
saleor/dashboard/group/filters.py	100% <0%> (ø)	⬆️
... and 7 more

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