Instead of showing start & end inputs show duration input element, explicit start & end are still present under tabs, where datatime picker is inlined (so it doesn't require any click to show controls). Tabs will also show desription of relative start and end, so it should be more readable what is the real duration of a silence
This commit turns label checkboxes into multiselect dropdowns that are populated from all labels of all instances of current alert (matched by alertname label). There is also a cocollapsible preview box that allows user to inspect raw silence JSON object we will send to the Alertmanager API. Fixes#69
