Fix cropped focus outline style for sidebar buttons

[SiobhyB]

Related to #82 (review)

Proposed Changes

• The focus outline style for the sidebar buttons is slightly cropped on the left. The reason for this is that the changes in Ensure only bordering areas of app are draggable #82 removed the button's left margin, leaving no longer space for the extra border to be displayed.
• With this PR, a small amount of left margin is added back to the buttons (this is then subtracted from the main sidebar).

Testing Instructions

• Navigate through the app using your keyboard and verify that the focus outline style appears as expected, no longer cropped.
• In addition, verify that it's still possible to drag the left border and that the amount of space it's possible to drag isn't noticeably smaller.

Before	After

Pre-merge Checklist

• Have you checked for TypeScript, React or other console errors?

[dcalhoun]

Thank you for addressing this! 🚀