Investigate local state for layout

[blondejamtart]

Currently, the layout attribute does not have a local state, and as such can only be updated by receiving an update from malcolm.
Adding a local state would allow for optimistic updates (i.e. deleting links) and also allow for users without write permissions (i.e. remote users) to move blocks on their local layout so they can actually see them

[blondejamtart]

Whilst flowgraph attributes don't have a local state, the current/last viewed layout does have one. Modifications have been made to the autolayout code to allow for this to be run only on the local state, it just needs to check an appropriate flag (i.e. writeable). Similar modifications to the move block action are needed. Fully "locking" the layout should not be done any more, but deleting/adding blocks and links should remain disabled. (N.B. this investigation was done as part of #450; deleting links now optimistically updates the layout local state)