more plenum fixes

items in the pr:

1. added parse_future_plenum_meetings job that parses knesset website for date and times of future plenum meetings.
2. added support to display members that were in a plenum meeting, this requires a job that parses those members from the protocol, there is a job that does it (update_persons_in_cms) and it works well on my local environment but I think it's an old one, should ask Ofri if it is ok to use it. anyway, this commit is safe to merge even if there are no members in any meetings.
3. add support for plenum description - someone needs to actually write the description in the admin interface under committees > plenum committee > description. if there is no description there will be no change in the display.
4. fix title of "all plenum meetings" page

after merge:

1. run "./manage.py parse_future_plenum_meetings -v2" - see if all is ok
2. add cron job "./manage.py parse_future_plenum_meetings" to run once per day
3. (optional) add a description for plenum in the admin interface (committees - plenum - description field)