Add LibreTranslate as an option to External Services

What does this MR do?

This adds LibreTranslate as an option to the External Services list.

It has the URL field as well as API Key field (in case people want to use libretranslate.com or want to use API Keys on their own/shared Libre Instance)

Screenshot showing example translation from LibreTranslate:

Closes #807 (closed)

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