Add icon to MR popover

Context

Context: #448868 (comment 2117391060)

There are quite a few popovers on GitLab. Most of them use an icon to the left of the popover detail content, but the MR pop over does not.

Popover Element Screenshot

MR

popover-mr.png

Issue

issue.png

Epic

popover-epic.png

Username

@mcbabin

popover-user.png

Proposal

Add the MR icon to the left of popover detail content in the MR popover.

popover-mr.png

Edited by 🤖 GitLab Bot 🤖