Improve the UX of the message editor in the headers tab

[SzymonPobiega]

Describe the suggested improvement

Is your improvement related to a problem? Please describe.

Improve the UX of the message editor in to ensure it is both safe and enables power-users to make advanced changes to the headers/contents.

Describe the suggested solution

Depending on the source of the message, some header modification should locked by default, but it should be possible to "unlock" them.

Describe alternatives you've considered

Additional Context

Currently the locking of editor fields is based on the MessageIntent header. If it is present, all NServiceBus-related headers are locked and there is no way to "unlock" them. This behavior was supposed to prevent mistakes that can render messages unsuited for processing but is likely to restrictive for the power users.