CORE-5832 Added new App create and copy endpoints.

Added POST /apps and /apps/:id/copy endpoints.
Updated Donkey endpoints and docs with new endpoints.
Refactored some common functions out of metadactyl.zoidberg.pipeline-edit and into metadactyl.zoidberg.app-edit.
CORE-5832 Fixed pipeline create, copy, and edit endpoints.

Fixes for bugs introduced by commits 6d4f040f25c5dc9a14e158d6a979f06f9f2c34fb and 47b9cf431a799939232e762446d4133841d5a14f
CORE-5832 Added paging params to /apps/categories/:id