Add CI workflow task to detect new strings

[kaedroho]

This PR adds a new Github workflow task that checks if there are any changes to translatable strings that haven't yet been extracted into a PO file.

I've set up a one-way link from this repo to Transifex so that translations are updated there as soon as we've generated the PO files here.

[kaedroho]

@zerolab

This PR adds a Github actions check in to alert us if we merge anything that requires the PO files to be rebuilt.

The reason why I haven't merged it is just that I thought it needed to display a more useful message of the check failed.

Happy to add this in if you think this is still worth having.

[zerolab]

This is certainly useful. I agree that it could use a more meaningful message. AWill this fail if there are new messages? so it is more than just an info blurb?

[zerolab]

I think I will close this now as the workflow we had so far worked well in my opinion. Thank you for this (and all the other PRs) Karl