Format pinned message changes to reflect the actual changes

[ShadowJonathan]

Is your suggestion related to a problem? Please describe.

When looking at #17938 I realised that it might be hard to see what the user pinned/unpinned exactly.

Describe the solution you'd like.

Something similar to discord, which shows that a user has pinned a specific message (the "a message" is clickable);

It should look at prev_content and extrapolate which message was added/removed. If the change is more than just 1 message addition/removal, it should fall back to the previous behaviour (of nondescript "changed the pinned messages")