Update notifications settings layout

What does this MR do and why?

• Moves notifications settings into single column
• Separate groups and project settings into separate columns
• Remove notification icon on each project/group (it's duplicated in each dropdown below)
• Removes some potentially unnecessary help text

Screenshots or screen recordings

Before	After

How to set up and validate locally

Profile > preferences > notifications

UX Paper Cuts 16.2 → Settings UI (#398215 - closed)