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
Drop haml templates in favor of underscore. Haml templates are harder to maintain and require extra dependencies, we already have underscore.js included and it provides a fast templating engine. Rewrite all client side templates using it.
Performance with underscore is pretty much the same as with haml (with 10k alerts).
