Introduce message contexts in l10n service

[arm1n]

In order to use same message ids we should introduce context based messages. Short example:

#: some/source/code/file.js
msgctxt "Base units"
msgid "KG"
msgstr "kg"
⁠
#: other/source/code/file.js
msgctxt "Countries"
msgid "KG"
msgstr "Kyrgyzstan"

This will not only involve l10n service, but also new template helpers as well as extending the extractor.

[arm1n]

This implementation is currently blocked by xgettext-template not supporting necessary keyword argument format for msgctxt. As soon as there's a solution without necessity to make a fork this feature can be implemented.