[milo] Refactor all html knowledge out of backends.

milo/frontend/view_frontpage.go

 // Copyright 2016 The LUCI Authors.
 //
 // Licensed under the Apache License, Version 2.0 (the "License");
 // you may not use this file except in compliance with the License.
 // You may obtain a copy of the License at
 //
 //      http://www.apache.org/licenses/LICENSE-2.0
 //
 // Unless required by applicable law or agreed to in writing, software
 // distributed under the License is distributed on an "AS IS" BASIS,
@@ skipping 61 matching lines @@
                 },
                 "error": "couldn't find ice cream",
         }
         return []common.TestBundle{
                 {
                         Description: "Basic frontpage",
                         Data:        *data,
                 },
         }
 }