Fixed issue boards sidebar close button with new navigation

Phil Hughesrequested to merge
issue-boards-close-icon-size into master

What does this MR do?

Fixes the close button for the issue boards sidebar displaying incorrectly with the new navigation turned on.

Screenshots (if relevant)

Chrome:

Before After
Screen_Shot_2017-07-20_at_14.36.22 Screen_Shot_2017-07-20_at_14.35.16

Safar:

Before After
Screen_Shot_2017-07-20_at_14.36.30 Screen_Shot_2017-07-20_at_14.35.16

What are the relevant issue numbers?

Closes #35296 (closed)

Merge request reports