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

spec/components/cooked_post_processor_spec.rb

@@ -1120,6 +1120,7 @@ describe CookedPostProcessor do
     end
 
     it "replaces large image placeholder" do
+      SiteSetting.max_image_size_kb = 4096
       url = 'https://image.com/my-avatar'
       image_url = 'https://image.com/avatar.png'
 
@@ -1134,6 +1135,7 @@ describe CookedPostProcessor do
       cpp.post_process
 
       expect(cpp.doc.to_s).to match(/<div class="large-image-placeholder">/)
+      expect(cpp.doc.to_s).to include(I18n.t("upload.placeholders.too_large_humanized", max_size: "4 MB"))
     end
   end