Remove the "Source" link at the top of PlasmaPy

[rocco8773]

Remove the link to the website's source code that appears in the top-right of the top-menu.

[StanczakDominik]

We could probably replace that with a link to the current file on the github repo and put it in the footer.

[rocco8773]

I could be on board with that. I think it would also be good to expand the content in the footer...maybe something like what GitHub has at the bottom of this page.

I know I have a habit of scrolling to the bottom of a web page, because the about, contact, etc. content is usually accessible there.

[rocco8773]

I removed the "Source" link in PR #80, but could not figure out how to move it to the footer. I figure Id leave this issue open because I still favor having the "Source" link in the footer over not.