i18n: merge translations in AngularGettextFilter

[jirikuncar]

• merge catalogs with same language and domain

It can be possibly done with an additional filter that produces merged PO files.

i18n = GlobBundle(
    catalog('invenio_messages_js', 'messages-js.po'),
    'node_modules/invenio_search_js/translations/*/LC_MESSAGES/messages-js.po',
    filters=['pomerge', AngularGettextFilter(...)],
    output=...
)

It needs some deeper investigation before naive implementation (hence Size: hard).