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
the first two points are covered in the file we are commenting on, the second two do not have to be covered in this file, from what I remember from my rails days you should not test basic rails functionalities like `update_attributes` ... and here I found functional test for code url https://github.com/kbrock/rubygems.org/blob/c3994fd11ecab28ea9b1629298a12eeb0be6a5bb/test/functional/rubygems_controller_test.rb#L123 ... at least it looks like related ... but this is not part of this PR, it should be already there, @sferik so does it look good?