JS code have a watchdog timer that will check if last data from unsee isn't too old, if that happens it will print an error in the UI and reload the page. This is a fail safe for weird bugs and cases where unsee is running unattended. Currently timeout for this watchdog is configured dynamically based on refresh rate, but if the backend is configured to pull data from Alertmanager less often than watchdog tolerance, then watchdog will trigger false positive. This commit changes watchdog tolerance to 15 minutes (it seems reasonable to assume people won't use Alertmanager pull interval longer than 1-5 minutes) and adds a note to the readme.
application/javascript was set in commit 5e501844a7 which works, but causes browsers to throw lots of errors in the console, as they try to parse those scripts as javascript, json type still works and doesn't have same issue with browsers
