BUG: Prevent users from undoing the initial document load with CTRL-Z.

Prior to this change, setting a document's initial content in the text
editor created a history entry, which meant that hitting CTRL-Z
repeatedly could clear the entire document.  This change clears document
history after loading so that repeated CTRL-Z applications return the
document to its state at load time.
Merge pull request #7046 from quantopian/fix-history-undo-load

BUG: Prevent users from undoing the initial document load with CTRL-Z.