Add a way to get cabinet membership data for a person

[chrismytton]

Problem

As part of #24 we'll need to get access to the unstable/positions.csv file in the legislature's directory in ep-data, at least until we move that data into the Popolo.

Proposed Solution

The code I wrote for the prototype does the job, so possibly just a case of pulling this into lib/everypolitician_extensions.rb and tidying it up a bit.

We should also have a Struct that represents each individual position object, so we don't have to use [] to access properties of a position.

Acceptance Criteria

I should be able to access the positions for a legislature in a Page::TermTable instance by calling house.positions.

I should be able to access cabinet memberships using the code in the README that was added in #15609.

Related Issues

• Executive memberships Executive memberships #24
• Work out how to display executive positions when known Work out how to display executive positions when known #13297

[chrismytton]

I've done a prototype of this in #15608 but it's currently blocked because the test fixtures have an old version of the position CSVs which is missing the type column. I've opened #15607 to track that.