Remove transferring badges

Anthrocon doesn't want the ability to transfer badges, so I've made the is_transferable property more customizable. It also now removes the "Transfer My Badge" button on the attendee confirmation page.
Two things about this which should probably be fixed:

1) You should make the default just say ``list()`` instead of ``list('')`` because the former is an empty list and the latter is a list containing a single element which is an empty string.

2) You'll actually need some extra code in ``config.py`` to make this work.  The problem is that this is a ``string_list`` so let's say you configure it to contain ``attendee_badge`` and ``supporter_badge``.  So the code you've written in ``models.py`` will compare an integer constant like ``ATTENDEE_BADGE`` to the string values in this list.

This is something we deal with by special-casing certain config options in ``config.py`` e.g. look at what we're doing for ``BADGE_RANGES``: https://github.com/magfest/ubersystem/blob/master/uber/config.py#L184

This is kind of kludgy, but basically what we do is take the string and convert it to the constant.  So we'd write something like

```python
TRANSFERABLE_BADGE_TYPES = [globals()[_bt.upper()] for _bt in TRANSFERRABLE_BADGE_TYPES]
```

In case it's unclear exactly what that's doing, we're iterating over the list, taking everything in the list and converting it to upper case, then looking up its value in the global module namespace.  So we'll end up with the integer constant values of e.g. ``ATTENDEE_BADGE`` instead of just the string "attendee_badge".