Add `pagination_class` for `candidates_elected` endpoint

[Muzer]

This enables use of the page_size parameter to control pagination.

Fixes #2402

### PR Checklist
- [ ] Update changelog (ie https://keepachangelog.com/en/1.0.0/) — err I can't find the changelog
- [x] References a specific issue or if not, describes the bug or feature in detail
- [ ] Note what card in Trello this work relates to — I have no idea where your trello board is
- [ ] Instructions for how reviewers can test the code locally — I have no idea how to test the code locally
- [ ] Tests have been added and/or updated — N/A
- [ ] Screenshot of the feature/bug fix (if applicable) — N/A
- [x] If any new text is added, it's internationalized — N/A
- [x] Any new elements have aria labels
- [x] No unintentional console.logs left behind after debugging
- [x] Did I use the clear and concise names for variables and functions?
- [x] Did I explain all possible solutions and why I chose the one I did? — Don't you Pythonistas always say "there's only one way to do it"? ;)
- [X] Added any comments to make new functions clearer — None needed
- [ ] Did I added or updated any new dependencies? Explain why.
- [ ] Did I update the package.json and/or Pipfile.lock version if relevant?
- [X] Have I rebased with the latest version of master?
- [ ] Added PR labels
- [ ] Update any history/changelog file
- [ ] Update any documentation
- [ ] Update dev handbook

[Muzer]

Build failure seems to be nothing to do with this change AFAICT?