Bug 34964: Add descriptions for different HTML customization regions

Bug 34964: Add descriptions for different HTML customization regions

This patch adds dynamically-shown hints on the HTML customization form
when the user selects a display location. The hint describes where the
content will be found.

Much of the descriptive text in this patch is adapted from the original
options' system preference descriptions.

To test, apply the patch and go to Tools -> HTML customizations -> New
entry.

- The page loads with the first display location preselected, so you
  should see a hint for the "ArticleRequestsDisclaimerText" option.
- Test the process of selecting each different display location option,
  confirming each time that the correct description is shown.

Signed-off-by: Barbara Johnson <barbara.johnson@bedfordtx.gov>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>