Use timestamp as cache-buster instead of localised date string

The nav-app adds a unique request-parameter to the URL as a cache-buster. If the CMS does not provide a cache-buster key, one is generated by the nav-app. Currently, this is done by calling new Date().toLocaleString() which produces something like "08/01/2021, 09:42:29" depending on the user's locale. A much simpler and less error-prone approach is to use the numeric timestamp returned by Date.now() which looks like 1610095486648.

Apart from this, the related unit test should be updated as currently, it compares values of toLocaleString() which will fail every once in a while.