#16 Make gridsize of memor activities configurable

[Chris_147]

At the moment the memory activities have fixed gridsizes: self.level_layout = [(4,2),(4,3),(5,4),(7,4),(6,5),(6,6)]
However, I think it is better to make this configurable because the progression can be difficult for smaller children and the first levels can be too easy for bigger children.
A possible solution could be to add a column in activity_options table for each memory activity where the user can store these gridsizes.
An extra would be to add an item to the admin gui so that this easy to do for the user.