Problems using 'week' and 'month' modes

[eagoetz]

I have found a couple of problems related to the week and month modes:

1. For week: if start-of-week is provided in the configuration file, then week will only work if the YYYYMMDD date specified is actually that day of the week, otherwise an error is thrown. If no start-of-week is provided, then the error is not thrown.
2. For week: linking to other days does not work in the calendar. An error message is provided when clicking on other days: "ERROR: Cannot format new date. If the problem persists, please report this at https://github.com/gwpy/gwsumm/issues/"
3. For month: clicking on other days of the month that has been generated brings you back to the start of the month (which I think is reasonable), but if you click on another month, it just brings you back to the original month