Updated the app controller to not send back buttons in most cases and to only provide next buttons when relevant.
Updated the enumeration of states to have labels so that buttons can have informative labels that cannot get out of sync with their action.
Modified the stimulus screen to have dedicated button styles.
Provided a new style to the next button that makes it more clickable and visible. 
Made more use of the abstract presenter to reduce duplicate code.
Added an option to retry if registration fails.
#67
#61
#58