Add Libre Translate support

Created by: supakaity

This PR adds an additional option for sites that wish to incorportate a Libre Translate backend into their server.

Currently the only option available is a paid (freemium) subscription model through DeepL (/Pro) and this also requires you to sign up with a credit card, and only with a credit card from a handful of seemingly arbitrarily "accepted" countries. If you don't have a credit card, or an acceptable one, then you cannot even get the free tier of API access, which even then is quite strictly limited (500k characters/month or about 2,000 entire posts a month) to try and force you to upgrade to the paid service. With a reasonable sized instance, this could easily exceed the free tier allowances.

So the option for a free alternative (which could be bundled into the docker image etc) can be seen as a good out of the box solution, with the ability to switch to a more accurate/paid solution as an option later.