Add "Changelog" field for metadata

As suggested in fdroiddata#113 (closed) I'd like to add this field. The current practice is to include the link at the end of the metadata's description field works for now, but makes it hard to access it directly. Once this field is available, changelog information will continue to be in the description field until it's available in the client -- which might take a while since I am not sure how the client should present this field.

This field is a link to an overall changelog file, it addresses not per-build changelogs which would be nice to have, but requires to have actually people/maintainers that have time to look those changes up. Also it's possible (and is set as default for github) to use the commitlog here, a lint --pedantic warning is thrown when the url contains "commit".

Please note that -- while I tested it somewhat locally -- this is my first change. I just tried to redo everything the issuetracker field is doing, I hope I didn't forget anything. As I am not aware of any test suite, please take your time to review.

If we need to discuss this matter further, I can re-create the issue from fdroiddata in fdroidserver as well.