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
I’m confused. There should be specs for each of these scenarios so we can be confident everything works (and continues to work) the way we expect:

* No URL data in the database, new gem is pushed with metadata (expect: gem metadata is displayed for this gem)
* URL data exists in the database, new gem is pushed with metadata (expect: ???)
* No gem metadata exists, user adds URLs via web UI (expect: URLs are displayed for this gem)
* Gem metadata exists, user updates URLs via web UI (expect: updated URLs are displayed for this gem)