Display postage everywhere we display a letter

To avoid the problem of having confusing defaults, the postage is now
set explicitly on every template.

Putting the postage ‘inside’ the letter template makes the interaction
for changing it consistent with how other parts of the template are
added.

Plus everyone loves skeumorphism.

app/assets/images/envelope-2nd-class.svg

@@ -0,0 +1,25 @@
+<?xml version="1.0" encoding="utf-8"?>
+<!-- Generator: Adobe Illustrator 21.1.0, SVG Export Plug-In . SVG Version: 6.00 Build 0)  -->
+<svg version="1.1" id="Layer_1" xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink" x="0px" y="0px"
+	 viewBox="0 0 277.3 233.2" style="enable-background:new 0 0 277.3 233.2;" xml:space="preserve">
+<style type="text/css">
+	.st0{fill:#B7A47E;}
+	.st1{fill:#C4B186;}
+	.st2{fill:#2B271D;}
+	.st3{fill:#FFFFFF;}
+	.st4{fill:url(#SVGID_1_);fill-opacity:3.000000e-02;}
+</style>
+<rect y="0" class="st0" width="277.3" height="233.2"/>
+<rect x="0" y="3" class="st1" width="274.3" height="230.2"/>
+<path class="st2" d="M62,28.2v148.2h189.7V28.2H62z M241.7,165.9h-86.3c0.2-9,2.2-17.3,5.8-24.8c3.5-8.4,11.9-17.3,25-26.7
+	c11.4-8.2,18.8-14,22.2-17.6c5.2-5.5,7.7-11.5,7.7-18.1c0-5.3-1.5-9.8-4.4-13.3c-3-3.5-7.2-5.3-12.7-5.3c-7.5,0-12.7,2.8-15.4,8.4
+	c-1.6,3.2-2.5,8.4-2.8,15.5h-24.1c0.4-10.7,2.3-19.4,5.8-26c6.6-12.6,18.4-18.9,35.2-18.9c13.3,0,23.9,3.7,31.8,11.1
+	c7.9,7.4,11.8,17.2,11.8,29.3c0,9.3-2.8,17.6-8.4,24.8c-3.7,4.8-9.7,10.2-18,16.1l-9.9,7c-6.2,4.4-10.5,7.6-12.7,9.6
+	c-2.3,2-4.2,4.3-5.8,6.9h55v22H241.7z"/>
+<polygon class="st3" points="277.3,233.2 0,233.2 0,0 "/>
+<linearGradient id="SVGID_1_" gradientUnits="userSpaceOnUse" x1="67.975" y1="51.415" x2="142.925" y2="126.365" gradientTransform="matrix(1 0 0 -1 0 233.89)">
+	<stop  offset="0" style="stop-color:#FFFFFF"/>
+	<stop  offset="1" style="stop-color:#000000"/>
+</linearGradient>
+<path class="st4" d="M199.3,165.9h-43.9c0.2-9,2.2-17.3,5.8-24.8c0.7-1.6,1.6-3.3,2.6-5L62,50.5v126h149.9L199.3,165.9z"/>
+</svg>