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/cooked_post_processor.rb

@@ -142,7 +142,15 @@ class CookedPostProcessor
     span = create_span_node("url", url)
     a.add_child(span)
     span.add_previous_sibling(create_icon_node("far-image"))
-    span.add_next_sibling(create_span_node("help", I18n.t("upload.placeholders.too_large", max_size_kb: SiteSetting.max_image_size_kb)))
+    span.add_next_sibling(
+      create_span_node(
+        "help",
+        I18n.t(
+          "upload.placeholders.too_large_humanized",
+          max_size: ActiveSupport::NumberHelper.number_to_human_size(SiteSetting.max_image_size_kb.kilobytes)
+        )
+      )
+    )
 
     # Only if the image is already linked
     if is_hyperlinked