Make Sponsor Text Clickable Links

Provide clickable links for sponsor text for accessibility.
Add titles to sponsor text links.
Change CSS ids of sponsor divs to reflect unique sponsor.
Remove excess white space.

Addresses comment by @hron84 on [issue #524](https://github.com/rubygems/rubygems.org/issues/524#issuecomment-13383956). /cc @dwradcliffe