Add an accessibility control panel

[skleinfeldt]

To support ATAG 2.0 Part B compliance, Plone admins need the ability to force editors to create accessible content. This includes requiring editors to enter contextual alt text for images and to run accessibility checkers on their pages. Create a new control panel allowing admins to enable/disable the following settings:

[ ] Force editors to enter alt text for images (includes Images, lead Images, custom types that use the lead image behavior, etc.) (#2837, #2838)
[ ] Force editors to enter contextual alt text for images used in rich text (#2836)
[ ] Force editors to run the accessibility checker on their content (#2833)