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/lib/upload_creator_spec.rb

@@ -114,6 +114,21 @@ RSpec.describe UploadCreator do
       end
     end
 
+    context "when image is too big" do
+      let(:filename) { 'logo.png' }
+      let(:file) { file_from_fixtures(filename) }
+
+      it "adds an error to the upload" do
+        SiteSetting.max_image_size_kb = 1
+        upload = UploadCreator.new(
+          file, filename, force_optimize: true
+        ).create_for(Discourse.system_user.id)
+        expect(upload.errors.full_messages.first).to eq(
+          "#{I18n.t("upload.images.too_large_humanized", max_size: "1 KB")}"
+        )
+      end
+    end
+
     describe 'pngquant' do
       let(:filename) { "pngquant.png" }
       let(:file) { file_from_fixtures(filename) }