update report style declaration to ease customization

Moves wrapping `<style />` to `report.style` template. With this change you can easily override styles for report like this:

```
<template id="my_style" inherit_id="report.style" >
    <xpath expr="//style" position="after">
      <link href="/my_reports/static/css/styles.css" rel="stylesheet"/>
    </xpath>
</template>
```
You touch just the template you need.