Prevent deletion of current droplet

[philippthun]

DropletsController: check if droplet is in use (i.e. app's current droplet) and raise an ApiError (UnprocessableEntity)

Prepare for the introduction of a foreign key from apps.droplet_guid to droplets.guid:

• DropletModel: handle ForeignKeyConstraintViolation and translate into ApiError (UnprocessableEntity)

• AppModel: handle ForeignKeyConstraintViolation and translate into ValidationFailed error

• DropletDelete: move destroy before other operations (deletion from blobstore, recording of event) so that an exception stops the control flow

• AppDelete: clear droplet_guid (i.e. current droplet relation) before deleting subresources (incl. droplets)

• AppAssignDroplet: handle ValidationFailed error and translate into already existing error

• Adapt tests to also work with a foreign key

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