Mark translatable texts

[lslezak]

Problem

• The texts in the web UI are not marked for translation

Notes

• This is just the first round, it does not cover all files, let's do it in smaller steps
• Adjusted building the POT file using xgettext, originally copied from the Cockpit code but make it better:
  • Use the JavaScript format (it betters parses the code and does not add the c-format marks)
  • Define only the keywords used by Agama (e.g. we do not use contexts)

[coveralls]

coverage: 72.14% (-0.003%) from 72.143% when pulling 4a58c38 on web_translations into 459221b on master.

[imobachgs]

LGTM