feat: mailbox icon in thread view

[madbob]

A proposal for #9535: in the thread view, I've added the icon of the parent mailbox for each message (and a tooltip with the full name of the mailbox itself).

Evaluating this feature, I've been concerned of adding the full name of the mailbox as it would eventually squeezes even more the envelope's header (especially on mobile). An icon is mostly decorative, slightly informative, and the tooltip provides the full information in unobtrusive way (in particular for non-special mailboxes not having their own recognizable icon).

Here I've also fixed a minor alignment issue with the sender line and message's preview line, and I've removed the mailboxId parameter for ThreadEnvelope (I've not really understood his utility, as the same information is carried by envelope and indeed the mailboxId binded to the local route in Thread impacts in the functionalities enabled for every child envelope).

Fixes #9535

[ChristophWurst]

Thanks for looking into this!

Would appreciate input from https://nextcloud-vue-components.netlify.app/#/Components/NcChip.

Maybe a NcChip? https://nextcloud-vue-components.netlify.app/#/Components/NcChip

[madbob]

Added a wrapping NcChip.

I would still avoid to include the textual name of the mailbox, both to save screen space and to avoid misalignment with different length names.

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