close button on issues board sidebar is too narrow

on this part of the page, on Firefox:

the close button (x) on the right of the issue name have problem:

• clicking anywhere on the right of the cross does not work
• on hover the cross does not get highlighted on hover (but it does for a fraction of a second when hovering stops)
• the cross might be too tiny

on chromium:

• the cross is bigger, maybe too big, or not centered horizontally properly
• clicking anywhere above or below the cross does not work
• same highlight problem on hover than on Firefox

Proposal

• Change svg size to 12px x 12px
• On hover, remove current light gray text color (we'll leave off the hover state until gitlab-design#579 (closed) is determined)