Don't show loading skeleton each time a known message is opened

[st3iny]

Is your feature request related to a problem? Please describe.

There is a loading skeleton each time a message is opened which has already been opened before. The data is available in the frontend and shouldn't change as we already do HTTP level caching on the api/messages/{id}/body response.

Describe the solution you'd like

The skeleton should only be shown if the message is not yet known.

Describe alternatives you've considered

No response

Additional context

No response