Allow adding multiple units per node

[acrollet]

When editing a node with BEE enabled, we should see a vertical tab that allows adding more units for that node. It should always be possible to add more units, but not to remove units. (since they might have bookings) The use case is to, for example, add a node called 'Canon 6D DSLR Camera' and be able to have multiple cameras.

[acrollet]

@nicolocaruso85 thanks, I see the form for adding units now and it reports that units are added, but when i reload the form it reverts to showing 1 unit, and there are no new units in the availability tab.

[acrollet]

Also, if I try to add units and also make a change to another field, I get this error when trying to save:

The content has either been modified by another user, or you have already submitted modifications. As a result, your changes cannot be saved.

[acrollet]

@nicolocaruso85 I can add units and they are created, thanks! Some more feedback:

• let's name the units with a number - e.g. {node title} 1, {node title} 2, etc.
• I don't see the additional units on the availability tab.
• I still get the 'content has been modified' error when trying to save other changes to a node.

[acrollet]

@nicolocaruso85 thanks, the first 2 issues are resolved. Is there any way to work around the 'content has been modified' error? thanks!