Pinning and unpinning a message should ask for confirmation

### Your use case

#### What would you like to do?

Pinning a message sends a state event to the room, notifying all room members that a message has been pinned. This action should require a confirmation, such as a modal pop-up to ask the user whether they're sure.

#### Why would you like to do it?

It's easy to mis-click the "pin" or "unpin" button, as it's right next to "edit" and other actions on the message hotbar:


#### How would you like to achieve it?

A modal pop-up, similar to Discord and other services, seems best.


### Have you considered any alternatives?

We could make it harder to find the button to pin things, but that seems counter-intuitive.

### Additional context

In Discord, if you hold down Shift while clicking on "pin/unpin", it skips the modal pop-up. This is a nice power-user feature.