Revert "Added global zoom level session variable"

This reverts commit 235f6de48c164d9e0875e999e1c260fb5efea579.
Remove Planner.fixedZoomByUser attribute

The attribute is removed as the zoom is going to be managed from the session.

FEA: ItEr77S15FilteringEnhancements
Get zoom from session in company Gantt view

Add a listener in order to update the value in the session when user changes the
zoom.

FEA: ItEr77S15FilteringEnhancements
Get zoom from session in project Gantt view

Add a listener in order to update the value in the session for each project when
user changes the zoom.

FEA: ItEr77S15FilteringEnhancements