Improve code structure of task exports

With TRANSLATE-2869 another task export was added.

Most of the export logic is already in separate classes, but the classes do not have a unified interface and a lot of code is still in the taskController it self.

This should be changed so that each export adds its context configuration and specific download methods to the task controller for usage instead of implementing it directly in the taskcontroller. This would also enable the possibility to easily add exports via plugin.

Description from duplicated issue TRANSLATE-3190:
Whole task export(context check and context set) logic takes big portion of the current task controller code. This should be moved to separate class.