Make Plain Post into a Static Application

[smcgregor]

The "plain post" endpoint should serve a completely untemplated application for creating markdown content. This way we could serve it from the extensions instead of the content server.

Other related tasks are:

• Making the current posts/new endpoint a selection screen for one of the posting applications (PrivlyPosts and ZeroBin)
• Add a list of posting applications to the right side of the ZeroBin application

We also need to consider creating a completely separate application for the post endpoint.

[smcgregor]

This is completed on the deprecation-cleaning branch and will be merged as soon as we publish the new priv.ly site.