Add alt text instructions to widget introductory text

[alundgard]

For widgets with an alternative text field and decorative checkbox, append the following instructions the widget introductory text:

For each item, please enter alternative text or appropriately check the decorative box. Guidelines for writing alt text.

Where “Guidelines for writing alt text” is a link that opens the guidelines in a new browser tab.

As the Spotlight default, link to general-purpose W3C alt text guidelines. Also, allow a custom link URL to be specified per Spotlight instance.

• [x] #2582
• [x] #2583

[dnoneill]

Kicking this back into ready. Needs to wait until a new release of spotlight.