Pin message/thread

[ChristophWurst]

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

As a Nextcloud Mail user I want specific messages to not move out of sight.

Describe the solution you'd like

Allow me to pin a message or thread, which will make it stay on top of the thread list.

In a technical sense we will sort by pinned first, then the actual sort attribute (date).

Describe alternatives you've considered

N/a

Additional context

N/a