Adds User functions on application
Refactors petitions/index to use the causes/list partial
Saves the last viewed petition in the session
Adds link to start a petition on the petitions/index view
Passes just the ID of the petition to the session
Grabs the last viewed petition's id, finds the petition, and passes it to the view
Renders petitions/last_viewed partial on petitions/index view page
Writes the petitions/last_viewed partial for use in the petitions/index view