Prevent duplicate stage rewards

• Fix #155: prevent task stage from giving duplicate rewards, based on old task progress
  • Normal (stages 1-4) and Repeat (stage 5) Stage buttons prevent multiple clicks, preventing confusion and unexpected rewards.
  • Stages do not give rewards for old-stages or those that are out of order, which can occur when a user has pressed the back-button on their browser (prevalent in Firefox and Chrome).
  • Both go-buttons and undo-buttons provide the user with a loading indicator (when necessary), giving them immediate feedback for their actions, hopefully preventing confusion.
  • Stages belonging to a task with a chain requirement will not give rewards if the user has failed to meet the chain requirement after having loaded the task. This occurs when the user has un-mastered or un-completed (based on the chain requirement) the required task, without refreshing the desired task. Such instances result in the user's browser being refreshed to display the desired task's current status (e.g. the chain requirement error).