Leverage metadata to populate gems urls

Lets have keys for the common urls we associate with ruby gem projects per #718

The README has defined `issue_tracker` as an example metadata entry. So I went with that.
There were a few examples in the wild with the source code defined as `source_url` - and other seemed to like this as well. It is unfortunate that these two are not consistent.

The others (`wiki_url`, `documentation_url`, and `mailing_list_url`) were my best stab. Don't think anyone really had much of an opinion. We can remove from this PR if you want. I added it since those fields are available in `rubygems.org`.

Thanks

/cc @qrush @fwolfst
@mpapis IMHO, the website data should always take precedence and gem metadata should be a fallback. User shouldn’t have to push a new gem version just to change the bugtracker URL. Users who never use the web UI will get the behavior you prefer. I think it would be very confusing and frustrating to make a change on the web UI, hit the “Save” button, and not see that change reflected on the website.