Add generics to many Lists; remove unused imports; add a FIXME

Using Eclipse I added type arguments to many variables and parameters. A few could not be changed because both `Dataverse`s and `Dataset`s are added to a raw `List`, e.g. in `DataversePage.getContents()`. I added a FIXME there, but the same problem can be observed elsewhere. I couldn't find uses of that code.

I also let Eclipse remove unused imports, because they are easily fixed and readded if necessary.