[MJ-121] & [MJ-122] Delete/Disable Account

Following changes:
- **New account settings page** - All the different account related settings were getting too unwieldy to handle as separate views listed in the topbar dropdown menu. Instead, I created an all encompassing account settings view that we can add onto in the future:
![account-settings](https://f.cloud.github.com/assets/666486/265103/1944307e-8da5-11e2-98cb-5eb5155eceee.png)
- **Delete account** - Ability to delete your user account permanently
- **Disable account** - Ability to temporarily disable your account. There is an additional is_disabled field for the user model now to accommodate this. You can still log in to your disabled account but have no access to any features except to send yourself an activation link. The reactivation process just uses the current registration model, it just regenerates a new key.

All tests pass.