Add pie chart of amounts by category

[ChristopherJohnston]

Initial implementation of #82. Need to work on the formatting and styling, plus addition of date ranges but it's a good start.

By Category

By Participant

[ChristopherJohnston]

To be useful as a summary, I think this also needs the ability to filter by month, between a date range, since last reimbursement, etc.

Export to pdf would also be good

[Maxco10]

Good idea.