[fix] widget/scrollhtmlwidget: default_font_size same as infofont and Screen:scaleBySize() (#4864)

Otherwise it's:

1. Too small on virtually all devices due to a lack of scaling.
2. Unusably small by default.

It's currently scaled by the callers (dictquicklookup & footnotewidget) based on other properties, but you shouldn't have to change it from the default just to get something usable. ;-)

frontend/ui/widget/scrollhtmlwidget.lua

@@ -18,7 +18,7 @@ local Screen = Device.screen
 local ScrollHtmlWidget = InputContainer:new{
     html_body = nil,
     css = nil,
-    default_font_size = 18,
+    default_font_size = Screen:scaleBySize(24), -- same as infofont
     htmlbox_widget = nil,
     v_scroll_bar = nil,
     dialog = nil,