JSON upgrades and optimisations

There are various improvements we can make to our JSON handling. These are the currently planned steps:

1. Introduce cop for JSON wrapper, disabled
2. Add JSON wrapper
3. Enable cop but not for old code Cancelled because it's the same number of changes as autocorrecting
4. Multiple smaller MRs to update all existing usage to use wrapper
5. Enable cop for everything (requires an issue for discussion first)
6. Update wrapper to handle old way of parsing JSON
7. Upgrade json gem from 1.8.6 to 2.3.0

I'll be linking the MRs to this issue.

Relevant feature flags

:json_wrapper_legacy_mode, default_enabled: true