feat: ability to cancel a draft

[madbob]

The ingenuous solution here provided is to explicitely trigger the draft-saving function immediately when the NewMessageModal is opened, so to move the whole component in the canonical "draft saved" state (displaying the explicit trash button).

Alternative solutions would imply a more complex integration with MessageService.deleteMessage() (instead of DraftService.deleteDraft()) when required.

Fixes #2492

[ChristophWurst]

Thank you for the PR

[ChristophWurst]

I'm hesitant about the mechanics of the solution because opening an existing draft and not making any changes should probably not even start the whole draft complexity.

So yeah, I think we need the alternative.

[madbob]

Superseeded by #13284

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