TVB-2661: Export/Import of projects from web GUI

Created by: liadomide

This PR continues #194 by touching remaining TODOs in the Import part, as well as refactoring related code.

Done here:

• On import from the old form with Operation.xml, create a ViewModelH5
• Links Import

Left for another PR (same task, same branch):

• ResultFigures op_gid reference
• check coverage and potentially add unit-tests
• Review the way ViewModel.gid is passed to a new Operation constructor. Maybe replace params with view_model_gid column directly ?

Left for another task:

• DataTypeGroups (export and import)