Improve translation handling

[lslezak]

Improvements

• Use the C language in the xgettext call (surprisingly handles JSX files better than the JavaScript language)
  • On the other hand the C language does not handle JS template string with backticks, that needs to be replaced
  • And multiline texts enabled in ESLint
• Move the xgettext call to a separate script so it can be run locally for easier testing
• Use C placeholder format (%s, %d) in the texts, that allows using the javascript-format attributes
  • That requires a trivial format-util library

This enables using the format attributes in the POT file:

#. TRANSLATORS: %s is replaced by a network connection name
#: src/components/network/ConnectionsTable.jsx:70
#, javascript-format
msgid "Edit connection %s"
msgstr ""

This should help with validating the translations.