View Stats needs next and previous sprint

[MalignantCarp]

So this bit of code here:

	showEndOfSprintStatsModal() {
		if (this.theSprint.isStarted()) {
			new EndOfSprintStatsModal(this.app, this.theSprint.getStats()).open()
		} else if (this.sprintHistory.length > 0) {
			new EndOfSprintStatsModal(this.app, this.sprintHistory[0]).open()
		} else {
			new Notice("No stats found to show!")
		}
	}

If there is no running sprint, then we show the first sprint in the history. So we'll need to either change that to show the last (I don't know if -1 is available like in Python?) or add a next a previous button to scoot through the different sprints.

[kinabalu]

Done.