Only allow branching a document from Master (Core)

At this moment, a branch is created from whatever branch is currently the unpublished variant. In the ciontent perspective this is not very confusing and a defendable choice.

However, it is confusing in the Channel Mngr. Assume the following scenario:

1. The unpublished variant for document X is the Halloween branch
2. You are looking in the Channel mngr at the Christmas project at document X
3. There is no Christmas branch of document X

The above means that in the Christmas channel, the Master/Core version of document X is shown. Then when starting to edit the document, there should be created a Christmas branch. If however the Christmas branch is created from the current preview, the visual editor (and the channel) would showthe Halloween version (while before it showed the Master/Core version). This would feel like an error.

Hence, we need the following

1) Branching is only allowed from Master
2) If the unpublished variant is not Master, branching is not allowed (in the content perspective)
3) Clicking 'Add to project' in the Channel Mgr in the context of Branch X in case the current unpublished variant is for Branch Y, should first checkout the Master, and then create the branch

For step (3) we most likely also need a DocumentWorkflow that tells which branch the current unpublished branch is for, since we do not yet have that. I'll create a separate issue for this