Fix bug where users assigned to a project could not be unassigned.

So that only newly assigned users will be sent a notification email,
the assign_users function was enhanced to only insert newly added users.

This resulted in assigned users who were not selected (i.e. to be unassigned)
being ignored and left assigned to the project.

The fix also checks for users who were assigned but are not selected, and removes
them from the list of users assigned to the project.
Merge branch 'master' of https://github.com/moexmen/okr

The conflicts were from fixing the same bug in unassigning users.

Conflicts:
	app/controllers/projects_controller.rb