ui/web: Set HTTP headers to prevent asset caching
ui/web: Set HTTP headers to prevent asset caching

This patch adds HTTP headers to each web asset response, disabling
caching for the payload. This prevents users running an old version of
the UI against a new version of Alertmanager once they updated. On the
down-side it increases page load size on subsequent page loads.

Signed-off-by: Max Leonard Inden <bb17a4a5e30bc4dd6ff6d9ba25e3b87c5052b918@gmail.com>

GoogleCloudPlatform/alertmanager,prometheus/alertmanager,prometheus/alertmanager,prometheus/alertmanager,GoogleCloudPlatform/alertmanager,GoogleCloudPlatform/alertmanager,GoogleCloudPlatform/alertmanager,prometheus/alertmanager