Use translated product descriptions in the backend

[lslezak]

Problem

• Return the translated product description from the backend
• Related to Generate and upload the product POT file to allow translating the product descriptions #851

Solution

• Find the translation matching the current locale
• When the translation is not available it returns the original (untranslated) description

Notes

• For some reason when changing the language in the web UI it is not propagated to the Software service. But propagating to the Storage and Manager services works fine. 🤔
• This need some more debugging 🐞

Testing

• Added a new unit test
• Tested manually (with a manually patched Software service switched explicitly to the Czech language, see the note above)

[jreidinger]

my wild guess here is that it won't raise signal when localization changes. So if UI cache list of base products, it will use it with old locale.

[coveralls]

coverage: 75.08% (+0.04%) from 75.039%
when pulling 62ed31c on product_translations
into 0f420c3 on master.