Advanced Styling codemirror errors

[SDScandrettKint]

If a user modifies the Advanced Styling in the UI, the style is saved as a formatted string:

It's not very pretty, but codemirror can load this and it render it as formatted 'json'. The issue for HER is that the tab and newline characters have all been removed, so codemirror is rendering the data exactly as expected - as a single line.

I tested Advanced Styling with properly formatted json by exporting a graph, cleaning up the data and reloading it. The data loaded as valid json in the database, but codemirror threw an error when I tried to load it in Advanced Styling. I suspect the issue isn't with codemirror, but with how it's being used. Unfortunately a fix for this (so that only valid json is managed) might mean cleaning up data that is not formatted as valid json and most likely means a migration to do it. For the time being @CWDamm-Kint's solution addresses the problem of being unable able to edit Advanced styling in cases such as Arches for HERs.

Originally posted by @chiatt in #11673 (review)