Add ability to choose between optimization types in the scheduling form

[talister]

The observing system and scheduler "now" gives the ability to choose between optimization in time (to get it observed sooner) or by airmass (to observe at the lowest possible airmass) with time being the default. This is controlled by a "optimization_type" : {TIME, AIRMASS} in the outer RequestGroup (alongside observation_note etc). The ability to choose between the types should be exposed to the user via the optional parameters section of the scheduling confirmation form, with a default of the current TIME.