Add maintenance script for deleting local passwords

Add maintenance script for deleting local passwords

This is mainly for the benefit of authentication extensions which
all need similar functionality for removing local passwords on a
wiki where local authentication was used for a while but has been
disabled, but can be used directly to just indiscriminately remove
the passwords of all users.

To test the change without irreversibly locking out users, an
option is provided to make the password invalid in an
easy-to-reverse way.

The immediate use case is I974184899c33.

This patch also introduces the maintenance/includes directory
to hold PHP files which are not executable scripts themselves.
(Previously such files had a .inc extension, but that is so PHP4.)

Bug: T57420
Change-Id: If7207b80a2c8374e90182e0b09d8f76ee94264b0