stylesheets: merge print rules with the application stylesheet

- Having a separate stylesheet for print rules is not a best practice
  (it makes an extra network request)
- It makes migrating to Sprockets 4 easier

app/assets/stylesheets/application.scss

@@ -5,4 +5,3 @@
 // = require ./fonts
 // = require leaflet
 // = require_tree .
-// = stub ./print.scss

app/assets/stylesheets/print.scss

@@ -1,63 +1,65 @@
 @import "colors";
 @import "fonts";
 
-.new-header,
-.sub-header,
-footer {
-  display: none;
-}
-
-.print-header {
-  display: block;
-  margin-top: 30px;
-  font-size: 30px;
-  line-height: 50px;
-  font-weight: bold;
-}
-
-body {
-  font-family: "Muli";
-}
-
-.subtitle {
-  margin-top: -20px;
-  color: $dark-grey;
-}
-
-th,
-td {
-  vertical-align: top;
-}
-
-th {
-  text-align: left;
-
-  &.header-section {
-    padding-top: 1.2em;
-    font-size: 1.2em;
+@media print {
+  .new-header,
+  .sub-header,
+  footer {
+    display: none;
   }
-}
 
-.messagerie {
-  .messages-list {
-    list-style: none;
-    padding-left: 0;
-    max-height: none;
+  .print-header {
+    display: block;
+    margin-top: 30px;
+    font-size: 30px;
+    line-height: 50px;
+    font-weight: bold;
+  }
 
-    h2 {
-      font-size: 110%;
-    }
+  body {
+    font-family: "Muli";
+  }
 
-    .person-icon {
-      display: none;
+  .subtitle {
+    margin-top: -20px;
+    color: $dark-grey;
+  }
+
+  th,
+  td {
+    vertical-align: top;
+  }
+
+  th {
+    text-align: left;
+
+    &.header-section {
+      padding-top: 1.2em;
+      font-size: 1.2em;
     }
   }
-}
 
-.message {
-  margin-bottom: 40px;
-}
+  .messagerie {
+    .messages-list {
+      list-style: none;
+      padding-left: 0;
+      max-height: none;
 
-.updated-at {
-  display: none;
+      h2 {
+        font-size: 110%;
+      }
+
+      .person-icon {
+        display: none;
+      }
+    }
+  }
+
+  .message {
+    margin-bottom: 40px;
+  }
+
+  .updated-at {
+    display: none;
+  }
 }