IDOC: Allow users and admins to edit user information, change password

I've exposed the user form in a way that allows users (and admins) to update user information, including the password.

Changes:
* User page now accepts an "id"; if it is "new", a new user is created; otherwise, user info is edited.
* When user info is edited, password is optional.  If password is specified, the password is changed--otherwise, stays the same.
* Exposed a button from the user's "account" page to "change password".  Can also change username (note that if changed, check for collision will occur)
* Cleaned up forms so that:
    * Don't show facility info (or link to change) unless there's more than one facility
    * For students, don't show group dropdown if there are no groups to select

Testing:
* Tested creating new students as admin, and through sign-up link
* Tested changing student info as admin, through user_management page
* Tested changing student username to an existing user (both facility and django)

![image](https://f.cloud.github.com/assets/4072455/1276538/9dc12a68-2e73-11e3-8d1c-1434fe08d4b5.png)

All links on the user info leads to edit user page:
![image](https://f.cloud.github.com/assets/4072455/1276539/aa7ec832-2e73-11e3-8476-87b398cb22a7.png)

Now change password button appears.
![image](https://f.cloud.github.com/assets/4072455/1276541/b33c5af2-2e73-11e3-97c4-5b10dd3032d4.png)