Rename "Define Needs" UI to "Volunteer Opportunities"

We've have some internal discussions about some of the confusing language in the extension. The term "Needs" makes sense only from the perspective of the volunteer coordinator, while "volunteer opportunity" makes sense from both the perspective of the coordinator and volunteer. Basically, we were using two words to talk about the same thing depending on who was doing the talking.

Making this change means:

• changing the navigation menu
• changing the heading in the modal
• changing the link on manage projects that launches the modal

Did I miss anything?