(Reissue of 4.0.3 due to packaging issues)

• Render help_text when set on FieldPanel, MultiFieldPanel, FieldRowPanel, and other panel APIs where it previously worked without official support (Matt Westcott)
• Update special-purpose FieldPanel deprecation message to add clarity for developers (Matt Westcott)
• Fix: Add back rendering of help_text for InlinePanel (Matt Westcott)
• Fix: Ensure that AbstractForm & AbstractEmailForm page models correctly pass the form to the preview context (Dan Bentley)
• Fix: Use the correct custom font for the Wagtail userbar (Umar Farouk Yunusa)
• Fix: Ensure that buttons on custom chooser widgets are correctly shown on hover (Thibaud Colas)