Clean up old and unused code

[symroe]

This is an issue that's being worked on, but I'm using this as a general task list that is likely to span a number of PRs.

It's initially a place to capture comments from #257.

### Tasks
- [ ] https://github.com/DemocracyClub/electionleaflets/issues/268
- [ ] https://github.com/DemocracyClub/electionleaflets/issues/269
- [ ] https://github.com/DemocracyClub/electionleaflets/issues/270
- [ ] https://github.com/DemocracyClub/electionleaflets/issues/271
- [ ] https://github.com/DemocracyClub/electionleaflets/issues/272
- [ ] https://github.com/DemocracyClub/electionleaflets/issues/273
- [ ] https://github.com/DemocracyClub/electionleaflets/issues/274
- [ ] https://github.com/DemocracyClub/electionleaflets/issues/275
- [ ] https://github.com/DemocracyClub/electionleaflets/issues/276
- [ ] https://github.com/DemocracyClub/electionleaflets/issues/267
- [ ] https://github.com/DemocracyClub/electionleaflets/issues/278
- [ ] https://github.com/DemocracyClub/electionleaflets/issues/302