Add option to lock board

[SelinaBuff]

Discussed in #3030

^{Originally posted by brandstetterm May 30, 2023}
I think it could be cool if a moderator could "lock" a board so that no more notes can be added / notes can't be edited anymore.

User Stories:

1. As a moderator, I want the ability to lock a board so that no more notes can be added, in order to prevent further contributions and maintain the current state of the board.

2. As a moderator, I expect a simple and intuitive interface to lock and unlock the screen. (I can imagine this being included as an icon (lock) on the right side of the moderator menu.)

3. As a user, I want to receive visual feedback when the screen is locked. It should be clear that no further changes can be made.

You can find the Design on Figma. Go on the left menu on "🧩 Components" and then to the section "Lock board":

[SelinaBuff]

Design is done ✅

[brandstetterm]

Backend has been implemented by #3745