Add flag to HTMLTextField to not persist submitted data

Add flag to HTMLTextField to not persist submitted data

Sometimes the submitted text is not expected to show up as default
when the form is redisplayed after an error; password fields are
the obvious example for this, but in some cases (e.g. two-factor
token) it is useful for a normal text field to act like that as well.

The patch adds a new 'persistent' flag to HTMLTextField, which
defaults to false for passwords and true otherwise.

Change-Id: If0a52f61aa061bbb55bfdc76321ace7d3eaed934