Added partial rendering throught javascript.
The goal is to be able to create through the admin panel any type of
partial and include them in any of your post.

The include pattern is to use <link rel='partial'
href='/my-partial-name' />

A small javascript snippet looks for those link and asynchronously load
them. Once loaded, it replaces the <link> tag with the actual partial.
This has the advantage that you can position your partial in your post
and the position will be kept.

This first commit has all the asynchronous logic but for now the
partials are coded in the project and cannot be modified in the admin
panel.
Added mailing list signup. Removed first ab test.
Because mailing list is better than pageview, I rather have a form up
and running on the proper post than increasing my page view for now.