Move code to label trees/projects/volumes/annotations modules

[mzur]

Currently biigle-core contains almost all of the controllers and migrations for the whole application. The label trees/projects/volumes/annotations modules almost exclusively just provide some special views.

Move all the code (including controllers, migrations) that belong e.g. to label trees to the label trees module (likewise the other modules). biigle-core wil only retain the very basic functions of scaffolding and user management.

Pros

• The mentioned modules will become more meaningful.
• biigle-core could be used as a basis for other projects (e.g. Jan was interested in a Biigle like app for MALDI images, or a Biigle for Video annotation).

Cons

• We have to work on something that won't result in any new features.
• We have to implement some solution that aggregates all server and API documentation over all modules. We could use this even now but the documentation of modules that is now unavailable is reatively small compared to the one that is available through biigle-core.

@dlangenk Any comments?

[mzur]

We won't do this for now as the architecture works good enough as it is and we don't have any resources to spare.