Statistics menu for admin

En el controller:
```ruby
@age_groups = @members.group_by { |member|
  case (age(m.user.date_of_birth.presence) rescue nil)
  when 0..17 then ' -17'
  when 18..24 then '18-24'
  # ...
  else 'Desconocida'
  end
}
@age_counts = Hash[@age_groups.map { |name, group| [name, group.size] }]
```
Lo mismo por el género...