FEATURE: Humanize file size error messages (#14398)

The file size error messages for max_image_size_kb and
max_attachment_size_kb are shown to the user in the KB
format, regardless of how large the limit is. Since we
are going to support uploading much larger files soon,
this KB-based limit soon becomes unfriendly to the end
user.

For example, if the max attachment size is set to 512000
KB, this is what the user sees:

> Sorry, the file you are trying to upload is too big (maximum
size is 512000KB)

This makes the user do math. In almost all file explorers that
a regular user would be familiar width, the file size is shown
in a format based on the maximum increment (e.g. KB, MB, GB).

This commit changes the behaviour to output a humanized file size
instead of the raw KB. For the above example, it would now say:

> Sorry, the file you are trying to upload is too big (maximum
size is 512 MB)

This humanization also handles decimals, e.g. 1536KB = 1.5 MB

lib/validators/upload_validator.rb

@@ -138,7 +138,10 @@ class UploadValidator < ActiveModel::Validator
     max_size_bytes = max_size_kb.kilobytes
 
     if upload.filesize > max_size_bytes
-      message = I18n.t("upload.#{type}s.too_large", max_size_kb: max_size_kb)
+      message = I18n.t(
+        "upload.#{type}s.too_large_humanized",
+        max_size: ActiveSupport::NumberHelper.number_to_human_size(max_size_bytes)
+      )
       upload.errors.add(:filesize, message)
     end
   end