Fix password_reset_confirm URL for django change.

Django changed the uid parameter to the password_reset_confirm view to be
base64-encoded instead of base36. This means our URL had to change a bit.
Trivial change.
Change "check-box" to "checkbox" in the docs.

The existing docs were using "check-box," which is wrong. I guess I
added this ages ago, but I'm not sure why.

This replaces all occurrences with "checkbox."