plenum scraping should move to knesset-data

it is very similar to committee meeting scraping and protocol parsing

### todo
* add the low-level code to knesset-data-python
* the code that fetches the list of plenum meetings + parses the documents and returns protocol parts
* add the data as part of the datapackage in knesset-data-datapackage
* update relevant DB models in knesset-data-django

### corresponding open knesset code
* https://github.com/hasadna/Open-Knesset/blob/master/plenum/management/commands/parse_plenum_protocols.py

Start by reading plenum/management/commands/parse_plenum_protocols.py in Open-Knesset, then inspect the knesset-data-python, knesset-data-datapackage, and knesset-data-django repositories. Done means the low-level meeting and protocol parsing is moved, the datapackage includes the data, and the relevant Django models are updated.

Written by the indexing model from the issue text.