Plan optimizations

[pgiraud]

Inspired from Despesz

It would be nice to offer the users the ability to link plans. A "add optimization" could be added to let users add a new plan linked to a first one.

The idea would be to add a "add optimization" on the interface. Then a new form would be created with a hidden input referencing the id of the previous plan. In the database, there would be a new "optimization_for" column.

In the future PEV2 could even show the difference graphically. See dalibo/pev2#377