Closes#550.
`loginWithPassword` now matches username or email in a case insensitive manner.
If there are multiple users with a username or email only differing in case, a
case sensitive match is required.
Although `createUser` won't let you create users with ambiguous usernames or
emails, this could happen with existing databases or if you modify the users
collection directly.
Because MongoDB does not support case insensitive indexes, we perform a case
insensitive query both before and after inserting a new user, removing the user
when we detect another matching user has been inserted in the meantime. This
leaves us with the theoretical possibility that a server crash could occur in
between the insert and the second query or remove. In that situation there
would be two accounts with a username or email only differing in case, so we
will require a case sensitive login.
Also allow accounts-password email templates to set mail headers, and
for the From override to be a function rather than a constant.
Fixes#2858. Fixes#2854.
