Add election description and image fields to models and API.

@DSASanFrancisco/portal-members

Add description text and image fields to the Election model and API.

I have:

• Included a summary of the changes
• Linked all relevant Trello tickets to this request

Trello tickets:

• https://trello.com/c/YOA0QVkF
• https://trello.com/c/MLE9zqYW