Capturing synonyms - often there is no "translation"

[matentzn]

Rather than requiring for every translation to have a corresponding annotation, we need to be flexible enough, at least for synonyms, to not require a value in the source_value field. We just need to be able to say: In my language, X is a synonym for the disease; not just, what is currently the case: this HP synonyms translates to than synonym in my language.

We should, however, keep that restricted somehow to ensure that the translations are faithful - we don't want other languages to start writing their own definitions which may or may not correspond to ours, thereby changing the entire meaning.

Thanks @drseb for suggestion.