Add Category sitemap

[Pacu2]

Close #1446
Categories are included in sitemap:

(Please mention all relevant issue numbers.)

(If your changes affect the UI, providing "before" and "after" screenshots will
greatly reduce the amount of work needed to review your work.)

Pull Request Checklist

(Please keep this section. It will make maintainer's life easier.)

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 #1447 into master will increase coverage by 0.02%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1447      +/-   ##
==========================================
+ Coverage    83.4%   83.42%   +0.02%     
==========================================
  Files         128      128              
  Lines        5791     5799       +8     
  Branches      664      665       +1     
==========================================
+ Hits         4830     4838       +8     
  Misses        803      803              
  Partials      158      158

Impacted Files	Coverage Δ
saleor/core/sitemaps.py	100% <100%> (ø)	⬆️
saleor/product/models.py	92.16% <0%> (-0.06%)	⬇️
saleor/search/backends/postgresql_dashboard.py	100% <0%> (ø)	⬆️
saleor/dashboard/search/views.py	90.9% <0%> (+0.9%)	⬆️

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[patrys]

I think we should also prevent search engines from indexing filtered category lists (maybe link their unfiltered versions as canonical).

[Pacu2]

@patrys already on it #1345