HTMLForm: Add "cloner" type

SecurePoll is going to need the ability to have a form for stuff like
"one or more admin usernames" and "one or more poll questions, each with
one or more options".

This change implements a generic field container that simply displays as
a <ul> followed by an "add more" button, with each <li> containing
various fields including a "remove" button.

Since this is only going to show up in SecurePoll to people creating a
poll (not to general users), the current design is functional but not
necessarily beautiful. Those interested in beauty are welcome to do so
in a followup change.

Change-Id: I46fad3971739ddc961259fe32eb6e1cd265a1c06