Allow check to ensure a message / set of messages from a file is present in other files

[jdforrester]

This in particular is looking to ensure that e.g. all the messages in en.json are also available in fr.json, or that the message is available in all languages, something that WMF's Fundraising are looking for.

Maybe something like:

sourceFile='en.json';
`completeFileTranslations=[ 'fr.json' ]```

… for the former and:

completeMessageTranslations=[ 'disclaimer' ]

… for the latter?