Publicize: Make custom message optional

#### Steps to reproduce the issue

- Make publicize connections to FB, Twitter, G+
- Add a new post, and click 'edit details' in the publicize section.
- Deselect the Facebook option and publish the post.

#### What I expected

I still wanted to use the default message when sharing, I only wanted to prevent sharing to Facebook, so I expected to be able to select whether to use the custom message or not.

#### What happened instead

Instead, once 'Edit details' is clicked, the custom message is used no matter what, and there's no way to go back to the default message.

So in effect, there is no way to just deactivate one (or more) publicize options without using the custom message.

Reported in 3113058-t

Start by reproducing the Publicize flow described: connect Facebook, Twitter, and Google+, edit the post details, deselect Facebook, and publish. Trace where editing details forces the custom message, then make the default message remain selectable when a network is deselected; verify that disabling one network does not require a custom message.

Written by the indexing model from the issue text.