Improve admin email notification messages

[Bravo555]

Look

We could use some CSS and JS to make the email look nice. We can throw some fancy buttons to click for accepting/rejecting a quote instead of plain links.

Working method

Right now, to accept/reject a quote we use plain links that make the browser do GET request to accept|reject_with_token and provide token as path parameter. As using GET request to make changes on the server isn't accepted by the spec, we should have only one endpoint for accepting/rejecting the quotes and make the email system fit to them somehow. Fortunately, with a bit of javascript, we can make just use fetch() to use correct method (I suppose).