multi-vcs: validate Project vcs_type against Application Settings
Now that we have the `vcs_types` Application Settings field, we use it to enforce upon Project creation that the selected `vcs_type` is among them. This validation has to be done by a separate method in order to access the `CurrentSettings` object. Also, we've considered making the default value for `vcs_type` in `CreateService` not to be the library-level default, so that explicitely passing a value is never required, even in the case where the library default is not allowed by Application Settings, but finally decided that failing with an explicit message in that case was better than such magic. Note also that the Web UI will always pass a value, so this case is in practice for API calls only.