Notifications dropdown on project page now has descriptions

## What does this MR do?

Adds descriptions to the notification settings dropdown on project pages. Also changed the order of them to make more sense.

## What are the relevant issue numbers?

Part of #12758

## Screenshots (if relevant)

![Screen_Shot_2016-05-31_at_09.35.24](/uploads/b30d82701a13ec15f672139e1225acf5/Screen_Shot_2016-05-31_at_09.35.24.png)

See merge request !4374