Articles/Status reports: Update deadlines

As discussed on developers@ the actual deadlines are not working.
It happens frequently that many reports are submitted past the deadline
and that publication gets stalled until all expected reports have
arrived.

Hence, from now on,

• we shall have a unique deadline on the 14th of the first month past

the end of the quarter;

• the deadline will be actually enforced: the full status reports shall

be published as soon as ready after the deadline. Reports that could not
make it within the deadline shall be moved (and possibly completed) to
the next quarter.

Reviewed by: adamw, Graham Percival <gperciva@tarsnap.com>
Pull Request: https://github.com/freebsd/freebsd-doc/pull/610