Clean up sectors for case studies and organizations

We’ll want to migrate existing case study categories into these. So need a mapping of every old category, like:

• Agriculture → Technology
• Arts → Sports & Entertainment
• Education → Education

Re-titling this so its more clear, this updates the sectors on the case studies themselves, not just the filters.

These options are also shared with sectors for organizations, like “Education, Enterprise, Government, Healthcare, Media, Non-profit, Telecommunications” on https://www.drupal.org/previousnext → are these new sectors good for organizations too?

And a mapping is in the Google doc in the issue summary. It would be good to move these into the issue summary for easier review.