Move double embed protection to embed.js

Logically, it makes sense to prevent double _embedding_ in the embed
code.

Also, there may come a time when it's acceptable and possible to
instantiate the application multiple times for different parts of a
document.

Finally, this prevents loading CSS and polyfills twice, as well.

h/static/scripts/hypothesis.js

-if (window.annotator) {
-    // Not deploying app, in order to avoid double deployment
-    return;
-}
-
 var Annotator = require('annotator');
 
 // Monkeypatch annotator!