Suggestion: allow toggling flight on plots

[MilesBHuff]

As a corollary to TownyAdvanced/Towny#7504, I'd like to request being able to toggle flight on plots just as we can currently toggle PvP or mobs on them. This is to help ensure that tfly rights do not unlevel the playing field of competitive non-PvP games. This change would help servers adhere to the new EULA + MUG.

Admins should also be able to toggle flight, using a subcommand of /ta plot.

Also: it is possible to implement TownyAdvanced/Towny#7504 by just having it be the case that setting PvP to "on" for a plot also sets flight to "off", and prevents the setting of flight to "on" until PvP is "off".