Import/Export New models and milestones errors while importing

Created from https://gitlab.com/gitlab-org/gitlab-ce/issues/21295#note_15998801

• We don't cope very well with 1 to 1 relations straight from project - project_feature is part of the params in the project initializer when it should be treated as a separate model and update the project later.
• Milestones and possibly labels require a certain order in order to get imported correctly. The following scenario may happen:
  1. Exported project that contains milestones associated to an issue and also unassigned milestones
  2. Unassigned milestones are then created successfully
  3. Assigned milestones get created, but then they fail when the milestone attempts to get assigned to a project once again

This will be fixed if we first create all the milestones, then for issues/MRs we assign the already created milestones (instead of the other way around)