[change/admin] Format configuration and configuration variables as indented JSON if the user has only view permission

[pandafy]

The JSONSchema editor is not shown when the user only has view permission. This also hides the configuration of the device. Moreover, the configuration variables are shown as string representation of Python object:

This is not good for UX. We shall show the value of these fields as indented JSON in the UI when the user has read-only permission.

This issue affect both template and device admin pages.

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This issue was first found in #840 (review)

[nemesifier]

Would be good to address this as well as #560 to improve read-only capabilities.