Add tooltip

There's been a lot of questions surrounding how participation is calculated. This is step one in making the calculation more transparent to the user.

Possible next steps:
* [ ] Link to detailed calculation page. The calculation page should show the user the numbers and how the calculations were made.
* [ ] Add indication that tooltip exists. Originally I tried adding an information icon to the right of the percentage, but that made it look messy. Investigate alternative ways to let the user know more information exists.