Add a login system

Users log in with their LDAP account (I hope), and then Pyboard uses that to provide access to things like the grades and submissions. This stops students from submitting things for other students, automatically detects which section they're in, only lets them edit their own email, enables administrator powers, etc.

#### Migrating / extra testing

* Remove `sections` file - it was only used on the submission page before, and now the section is autodetected based on the logged in user
* Add `admins` file - a list of LDAP usernames that have elevated privileges, one per line
* Verify that the LDAP log in system works
* Verify that assignment submissions work
* Verify that only administrators can view `/admin` and `/download/<section>/<assignment>`