Move replaceSelected() into contentscript.js

Decreases loading times (HTTP Headers etc)
Move resize > main.js, minify jquery.selection.js

Decreases loading times etc
WIP Scratch forum post dropdown

When line 9 is uncommented, a new button should appear on the editor for
scratch forum posts. I'll add more later.
Merge pull request #1 from comp500/master

Added more efficient, smaller code.