Add mailing configuration

[dominik-zeglen]

I want to merge this change because it adds mailing configuration.

Screenshots

Pull Request Checklist

1. All visible strings are translated with proper context.
2. All data-formatting is locale-aware (dates, numbers, and so on).
3. Translated strings are extracted to .pot file.
4. Number of API calls is optimized.
5. The changes are tested.
6. Type definitions are up to date.
7. Changes are mentioned in the changelog.

[codecov]

Codecov Report

Merging #222 into master will increase coverage by 0.09%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #222      +/-   ##
==========================================
+ Coverage   73.97%   74.06%   +0.09%     
==========================================
  Files         588      589       +1     
  Lines        6143     6161      +18     
  Branches     1033     1035       +2     
==========================================
+ Hits         4544     4563      +19     
  Misses       1417     1417              
+ Partials      182      181       -1

Impacted Files	Coverage Δ
src/components/ImageUpload/ImageUpload.tsx	90% <ø> (ø)	⬆️
src/siteSettings/fixtures.ts	100% <ø> (ø)	⬆️
src/theme.ts	100% <ø> (ø)	⬆️
...s/components/SiteSettingsPage/SiteSettingsPage.tsx	100% <100%> (ø)	⬆️
...torybook/stories/categories/CategoryUpdatePage.tsx	61.11% <100%> (ø)	⬆️
...torybook/stories/siteSettings/SiteSettingsPage.tsx	60% <100%> (ø)	⬆️
...onents/SiteSettingsMailing/SiteSettingsMailing.tsx	100% <100%> (ø)
...mponents/CategoryBackground/CategoryBackground.tsx	92.3% <0%> (+7.69%)	⬆️

---

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 5131c50...9cb5c4c. Read the comment docs.