446 new ontohub oms generalization

This is clean pull request without any mixed commit w.r.t indefinite article. 
According to https://groups.google.com/forum/#!searchin/plataformatec-simpleform/i18n/plataformatec-simpleform/v90OMr4O4LM/cBwiA5LO0hYJ there is no way to do this in the yml file. 
Usually, I'd suggest to edit the corresponding view and add a `t`-call, but here we don't have any view with `source_url` that we actually use. This i18n key could only be used in the bulk upload which we dropped quite a while ago. It's a code-zombie, isn't it? (As well as everything that uses `source_url`)