Disallow svg images to be erroneously uploaded

Laravel's `image` validation rule allows svg files to pass validation, but we can't handle svgs so it would result in an unspecified 500 error which isn't nice.

src/Core/Validator/AvatarValidator.php

@@ -15,7 +15,7 @@ class AvatarValidator extends AbstractValidator
     protected $rules = [
         'avatar' => [
             'required',
-            'image',
+            'mimes:jpeg,png,bmp,gif',
             'max:2048'
         ]
     ];