Automatically load last expDef for each subject

[kevin-j-miller]

MC would be easier to use if it didn't require the user to know which expDef each mouse should be run on. When the user selects a mouse from the dropdown, it should automatically load the expDef that the mouse was last run on. It should still give the user the option to change that expDef, though, or to set the expDef for a new mouse that's being run for the first time.