Add an "access denied" text

In certain situations, when access is denied to a VAB, we will want to show some text to explain to the user why access is denied. For example, if the add button is for subscribing to content (literally, adding a subscription), and the user doesn't meet a certain criteria, we may want to tell them how to meet that criteria.

This update will add an additional textarea, above the prefix/suffix textareas, with this option. If anything is set, the text will show when checkaccess() returns FALSE.