Add default icon size to `sprite_icon`

The suggested icon size used for SVGs is 16px and is the
most common use case for this helper. Adding the value as
default makes using the helper DRY-er and encourages
better UI.

app/helpers/icons_helper.rb

@@ -45,7 +45,7 @@ module IconsHelper
     ActionController::Base.helpers.image_path('file_icons.svg', host: sprite_base_url)
   end
 
-  def sprite_icon(icon_name, size: nil, css_class: nil)
+  def sprite_icon(icon_name, size: DEFAULT_ICON_SIZE, css_class: nil)
     if known_sprites&.exclude?(icon_name)
       exception = ArgumentError.new("#{icon_name} is not a known icon in @gitlab-org/gitlab-svg")
       Gitlab::ErrorTracking.track_and_raise_for_dev_exception(exception)

spec/helpers/icons_helper_spec.rb

@@ -48,8 +48,13 @@ RSpec.describe IconsHelper do
   describe 'sprite_icon' do
     icon_name = 'clock'
 
-    it 'returns svg icon html' do
+    it 'returns svg icon html with DEFAULT_ICON_SIZE' do
       expect(sprite_icon(icon_name).to_s)
+        .to eq "<svg class=\"s#{IconsHelper::DEFAULT_ICON_SIZE}\" data-testid=\"#{icon_name}-icon\"><use xlink:href=\"#{icons_path}##{icon_name}\"></use></svg>"
+    end
+
+    it 'returns svg icon html without size class' do
+      expect(sprite_icon(icon_name, size: nil).to_s)
         .to eq "<svg data-testid=\"#{icon_name}-icon\"><use xlink:href=\"#{icons_path}##{icon_name}\"></use></svg>"
     end