Purge unused components and assets task should support PyPi and Bower proxy repositories

Currently the task "Purge unused components and assets task" is not configurable against and no-op for PyPi and Bower format proxy repositories.

Expected

• PyPi and Bower proxy repositories should be selectable in the repository list shown on the task configiuration
• when the task is configured for All Repositories, PyPi and Bower proxy repositories should not be a no-op, but instead be processed