Upgrade nunjucks

There were some annoying bugs in the version we were using, including
broken behaviour for the `if` block in templates. Upgrading to the
latest version fixes some of those issues.
Add utility methods

* util.whitelist: create a whitelist object with an `exempt` method
* util.objWrap: higher-order function for wrapping a value in an object
* util.isEmail: test a string for email-ness.
Add template filter

Now that we have two classes of users, we can't just send everyone off
to /admin. This provides a filter so the frontend only needs to know
about the access level and the filter can determine the correct path.
Rename: routes/admin.js => routes/render.js

`admin.js` has always been a pretty poor name for this file. It's
focused entirely with rendering the frontend templates, so why not
`render.js`?
Frontend for issuing badges

This is a rather large commit containing the bare-bones necessities for
issuing badges manually. When a user signs in, we first check to see if
they are super user (as set by the environment variable). If not, we
check against each issuers' `accessList` to make a list of issuers that
a user has access to.

If a user has access to some issuers, we show the list of all the badges
related to those issuers (via their programs). The user is taken to a
screen where she can enter a list of email addresses and issue a badge
to each of them.