[UI] Feature: Localtime / Timezone option

[breca]

I have some users that would benefit greatly from the ability to create and view silences via the web UI without having to do time zone math and I think this would go a long way towards generally lowering the barrier to entry for the Prometheus/AM ecosystem.

I think that this would likely be best achieved by fishing the time zone information from the user's browser and enabled and stored on a per-user basis via the 'Settings' tab, similarly to the current 'First day of the week' option.