Add empty S scraper for the pages of committees

This mainly sets up the getProviderArguments() implementation.
Implement retrieval of seats data
Get the French and Dutch name of the committee
Work around a bug of the Senate website

At least one committee on the website of the Senate displays its lists of seats multiple times. Moreover, these lists are sometimes mixed with lists of members of the Chamber.

This simple workaround prevents loosing any previously saved data and allows to avoid unnecessary processing.
Add a new item to the list of committee roles

And fix the alignment at the same time.
Improve retrieval of seat data

This change makes the method less flexible, since it will not return arrays containing null values anymore when an anchor is not present.

This is done on purpose to prevent members of the Chamber to be returned, since those never have a Senate identifier nor, as a result, an anchor.

It also means that potential senators listed without an anchor linking to their page will not be returned, but it’s not expected to see this kind of things during normal activity periods of the parliament.
Retrieve political group for each seat
[refactor] Remove one indentation level
Add conditional statement

This change also renames variables to improve clarity.
[refactor] Extract node retrieval to a variable
Merge branch 's-committee-scraper'
Add abstract S scraper
Remove `use` declaration from S scrapers

This makes the scrapers reference the Senate asbtract scraper instead of the global one.
Remove property from S scrapers

This property is now centralized in the Senate abstract scraper.
Merge branch 'abstract-s-scraper'