Fix vertical badge alignment in sidebar nav items

What does this MR do and why?

Fix navigation badges alignment (#421955 - closed) by making the wrapping span a flexbox, too. This way, it only takes the height required for its content.

Without this, this span takes more height, throwing off the badge's alignment in the parent flex container.

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

Before	After

How to set up and validate locally

• Go to any project and check the Plan/Issues entry in the left sidebar.
• The counter badge on the right wasn't perfectly aligned, but it was only off by a few pixel, so zoom in 300% to spot the difference.
• Same for all other badges (like Code/Merge Requests counter)