(bug 4959) Whitelist github gists for embedding.

Github gist embeds come as a <script>, eg
<script src="https://gist.github.com/5711118.js"></script>

The src URLs come in at least four forms: gist, user/gist,
gist/commithash, user/gist/commithash.

https://gist.github.com/5711118.js
https://gist.github.com/rickscott/5711118.js
https://gist.github.com/5711118/8fe4cf179fb12a874ccb3a262358b60ddbd57b19.js
https://gist.github.com/rickscott/5711118/8fe4cf179fb12a874ccb3a262358b60ddbd57b19.js

The first two give the most recent revision of the gist;
the other two yield the gist at a specific revision.