Remove palette styles container from fronts

There has been some discussion about the old palette styles container, added in the fronts tool to provide additional CSS for thrashers on a page.
DCR rendered fronts do not need this container so once we have fully switched rendering over to DCR, with no need to revert back to frontend for anything, these can be removed.

Relevant conversations:
- [in WebEx general chat](https://mail.google.com/chat/#chat/space/AAAAG9rU0m0/C-UqVXv8fQ0)
- [in WebEx Skynet chat](https://mail.google.com/chat/#chat/space/AAAAPTsU18A/WAihG4D0eAM)

In order to do this, we would need to ask CP to remove these containers from the fronts and reflect this in the code so that we no longer take these containers into account in our rendering flow.

There are potentially some applications at the time of writing that would fail due to this change so this ticket is here to remind us to chat about it in the future when we are more confident that this wont negatively affect rendering.

## Prevent Fronts Falling Back To Frontend

To prevent fronts ever falling back to frontend, we need to a) migrate all frontend fronts features to DCAR, or b) deprecate any frontend features that DCAR will not support.

```[tasklist]
### Tasks
- [ ] Deprecate `json.html` thrashers
```