Change how we deal with multi-proposals barclamps in list of barclamps

Until now, when a multi-proposal barclamp had no proposal, we were just
displaying a Create button that created a proposal named "default". And
then, later on, we were allowing the creation of new proposals with
different names.

Since "default" is not a great name and people will likely want to
change this, we now directly allow creating proposals with any name.
Merge pull request #978 from vuntz/multi-proposal-ui

Change how we deal with multi-proposals barclamps in list of barclamps