Make changing and removing system notes more informative

Problem

I cannot see which milestone I removed.

Usecase

I accidentally removed the milestone from an issue.. the system note didn't help me out.

Proposal

Should state which milestone is unassigned 😉

CC/Call to actions

@victorwu @smcgivern (assigned this UX debt , but can almost be considered a bug I think)

System notes

• @victorwu added the milestone 10.1 just now
• @victorwu removed the milestone 10.1 just now
• @victorwu changed the milestone from 10.1 to 10.2 just now