Code fields save a string

[groenroos]

In the Configuration UI, the "code" field currently saves the object as a string, which is obviously not correct.

Even getting the code editor to work properly thus far took some doing. Feels like the vue-prism-editor is quite fragile, and as it can't take an object as a value, there needs to be some parsing to ensure any field with a code editor first stringifies its content for the editor, and then parses it into an object for saving. All the while making sure these transformations do not trigger reactivity. Also, only Vuex knows which fields are code fields, but the Configuration state needs to stay outside of Vuex to make sure changes aren't visible throughout the UI before they are saved. And some kind of validation might also be useful.