Clearly differentiate the unresolved and resolved thread in the MR overview tab
## Problem to solve
[Slack thread](https://gitlab.slack.com/archives/C01EMBKS5DW/p1620706503409800) @rdickenson brought up a great point that it is currently not clear at a glance which threads are unresolved and which are not. 
Also, the below threads are both resolved but the second one (from the `Changes` tab) is hard to find if it is resolved or not.

Example MR - https://gitlab.com/gitlab-org/gitlab/-/merge_requests/61185
## Proposal
- Having the :white_check_mark: mark in the header for consistency.
issue