Groundwork for integrating expenses

Added expenses and their relationship to needs.

Todo:
• Allow need_poster and church_admin to edit these. Add, delete, update.
• Have them all show up in the UI (use them to show the total financial needs, etc).
• Integrate the modal to see them all in a list (where you can do the editing etc).
• Make the documentation always be a link... or should we also have the ability to upload documentation?