Add site glossary/thesaurus

Multiple plugins introduce the concept of a glossary/thesaurus.

For consistency reasons, the site should globally define such a list: terms and their reference translation in other languages. The glossary is then sent attached to the jobs to make the translator follow the site standards.

This list might be initially created as a small set of words .. and it will be growing over time.
Since these words are not always used in their neutral form in text content, we cannot reduce the list to the words used only in a job.