Refactor Data Model

[kapil1garg]

This PR refactors the data model around 3 main abstractions: PracticeObject, IssueObject, and FollowUpObject. Closes #36

Also adds:

• Contenteditable divs for tracking notes. Closes #34
• Drag and drop between practices #32
• Fixes empty issues. Closes #31
• Pulls in context for follow-up practices suggested last meeting