Allow easier propagation of default context

Right now, when creating a DnD-enabled text field instance, you have to pick one context from the list of public context, with the default value being set to the default global context. This is all good, until the point site contributors ask the site builder to change the default context on every standard content.

At that point, you're now facing the not all that pleasant task of having to go through all of your text field instances, updating them to use the new default context, and if you're not a Scald developer and don't know about the hidden 'dnd_context_default' variable, you also have to take care of setting that context for newly created instances.

I think that whole process can be fluidified quite simply:

- Add a new option on the field creation form, '- Use the global default context -' and select that by default
- Remove the various call in mee.module that tries hard to fetch dnd_context_default, because there's already code to do that in the ckeditor plugin anyway
- Expose the dnd_context_default variable in admin/config/content/dnd

There should be no upgrade path issue:
- For sites upgrading, there will be no change by default ; site builders can change there fields to use the new context value
- Every time an atom is dropped into an RTE, the default context gets resolved, and then stored (either in the SAS or as an attribute of the div). Changing the default global context thus won't provoke unexpected change to user content, and only affect new drops. I think that's the sane thing to do, ensuring we don't garble user content.

Thoughts welcome ; I'll probably post a patch tomorrow.