Make pageblock forms more flexible

[nikolas]

We need to come up with a way to better customize pageblock forms. If you create a custom pageblock that needs to handle POST data, but you need to render your own Submit button(s) for (and possible your own <form>), there's no clear method to doing this. In the past, Susan has resorted to using JavaScript to make interface changes to custom PageBlock's forms, which shouldn't be necessary.

See discussion here: https://frontdesk.ccnmtl.columbia.edu/logs/2015/02/27/#2015-02-27T10:37:20.577444106-05:00