Improve user experience for avatar cropping

- Avoid incomprehensible errors on non-integer cropping values
- Set the default cropping area to 80%

app/assets/javascripts/profile.js.coffee

@@ -31,7 +31,6 @@ class @Profile
       setTimeout ( -> # The cropper must be asynchronously initialized
         $modalCropImg.cropper
           aspectRatio: 1
-          autoCropArea: 1
           modal: false
           scalable: false
           rotatable: false

app/uploaders/avatar_uploader.rb

@@ -10,8 +10,15 @@ class AvatarUploader < CarrierWave::Uploader::Base
 
   process :cropper
 
+  def is_integer? string
+    true if Integer(string) rescue false
+  end
+
   def cropper
-    if model.instance_of? User
+    is_compliant = model.kind_of?(User) && is_integer?(model.avatar_crop_size)
+    is_compliant = is_compliant && is_integer?(model.avatar_crop_x) && is_integer?(model.avatar_crop_y)
+
+    if is_compliant
       manipulate! do |img|
         img.crop "#{model.avatar_crop_size}x#{model.avatar_crop_size}+#{model.avatar_crop_x}+#{model.avatar_crop_y}"
       end