Summary email needs a width limit

[awesomerobot (Kris)]

I’ve restricted the width and centered the email content here:

https://github.com/discourse/discourse/commit/a5fbdba9d46232922f299ba18806b368d6c1da2b

From a test send:

I’ve used tables to restrict the width and center which should ensure it works with older email clients:

<table>
  <tr>
    <td></td>
    <td width="650"> all email content within this td </td>
    <td></td>
  </tr>
</table>

Note that a width set on a td functions more like a max-width, and content will still shrink to fit smaller devices.