Update cache control for web assets
Update cache control for web assets

The commit [1] introduced the `cache-control` header field with the value
`no-cache`. This enforces any cache to revalidate. Apparently this does
not prohibit *FireFox* from caching the assets. `no-cache, no-store,
must-revalidate` does by prohibiting to even save anything of the
request.

[1] c3850708c1f31aad1db9cda355698ecdd4755e39

joehandzik/alertmanager,tyrken/alertmanager,GoogleCloudPlatform/alertmanager,joehandzik/alertmanager,tinytub/alertmanager,tyrken/alertmanager,Aloomaio/alertmanager,tinytub/alertmanager,tyrken/alertmanager,GoogleCloudPlatform/alertmanager,Aloomaio/alertmanager,joehandzik/alertmanager,tyrken/alertmanager,Kellel/alertmanager,GoogleCloudPlatform/alertmanager,prometheus/alertmanager,tinytub/alertmanager,prometheus/alertmanager,Kellel/alertmanager,prometheus/alertmanager,Kellel/alertmanager,GoogleCloudPlatform/alertmanager,prometheus/alertmanager,Aloomaio/alertmanager