Allow admins to set LOCKDOWN, and force all users to log in to an account to view.

**Background**:
I believe we originally set up KA Lite to be completely open (no login required), because we wanted to err on the side of accessible.  The downsides of this are:

* Schools and users may not take advantage of the mastery-based learning approach by accessing the content while logged in.  In fact, *they may not even be aware of these features*.
* We still sync back very little data from users.

My experience in Gitwe is that at least some users are both informed by administrators on how to log in, and familiar with the login process.  

**Changes**:
I've implemented a `setting`, `LOCKDOWN`, that  when set to `True`, will require students to have an account, and to log in, in order to access resources.  If they try to access resources without being logged in, they'll be prompted to log in--which now, also has a link to `sign up`.

Currently, by default, `LOCKDOWN = False`; however, I would argue that it is generally in the best interests of the users *and* us to set the default to `True.