Subscribe to snapshot or released versions

Currently, when doing the initial import from OCL, the whole source is fetched using the zipped version of a source (latest release).

Any further update is done by using a different endpoint, which given the date and time of the last import, returns a list of changed concepts and mappings. In this mode we are subscribing to a SNAPSHOT version of a source i.e. once any single concept is changed on OCL, it will be instantly fetched and imported by the module.

When subscribing we need to give an option (radio button select on the configuration page) to subscribe to only released versions.

In such a mode any further update is done by asking for the latest released version. If the version is higher from the previously imported one, then do a full import of concepts from the fetched zip file. Only Concepts and Mappings, which changed in OCL should be imported from that zip file (already implemented).