Fix scroll offset on bugnote links

When clicking on a direct link to a bugnote, e.g. from "View Issue
Details" page or from a notification email, MantisBT did not account for
the Navbar's height, causing the upper part of the note to be hidden
behind it.

Adding scroll-margin-top CSS attribute allows proper positioning.

Fixes 0021675

Signed-off-by: Damien Regad dregad@mantisbt.org

Changes to original contribution from PR 2137

• Rewritten commit message, added bugtracker reference
• Coding guidelines