Use better reblog icon and improve contrast in HTML e-mails (#6272)

app/javascript/images/icon_cached.svg

-<svg fill="#FFFFFF" height="24" viewBox="0 0 24 24" width="24" xmlns="http://www.w3.org/2000/svg">
-    <path d="M19 8l-4 4h3c0 3.31-2.69 6-6 6-1.01 0-1.97-.25-2.8-.7l-1.46 1.46C8.97 19.54 10.43 20 12 20c4.42 0 8-3.58 8-8h3l-4-4zM6 12c0-3.31 2.69-6 6-6 1.01 0 1.97.25 2.8.7l1.46-1.46C15.03 4.46 13.57 4 12 4c-4.42 0-8 3.58-8 8H1l4 4 4-4H6z"/>
-    <path d="M0 0h24v24H0z" fill="none"/>
-</svg>
\ No newline at end of file
+<?xml version="1.0" encoding="utf-8"?>
+<svg width="2048" height="1792" viewBox="0 0 2048 1792" xmlns="http://www.w3.org/2000/svg"><path d="M1344 1504q0 13-9.5 22.5t-22.5 9.5h-960q-8 0-13.5-2t-9-7-5.5-8-3-11.5-1-11.5v-600h-192q-26 0-45-19t-19-45q0-24 15-41l320-384q19-22 49-22t49 22l320 384q15 17 15 41 0 26-19 45t-45 19h-192v384h576q16 0 25 11l160 192q7 10 7 21zm640-416q0 24-15 41l-320 384q-20 23-49 23t-49-23l-320-384q-15-17-15-41 0-26 19-45t45-19h192v-384h-576q-16 0-25-12l-160-192q-7-9-7-20 0-13 9.5-22.5t22.5-9.5h960q8 0 13.5 2t9 7 5.5 8 3 11.5 1 11.5v600h192q26 0 45 19t19 45z" fill="#fff"/></svg>

app/javascript/styles/mailer.scss

@@ -300,7 +300,7 @@ h3 {
 .footer {
   .column-cell,
   p {
-    color: lighten($ui-base-color, 26%);
+    color: lighten($ui-base-color, 34%);
   }
 
   p {
@@ -313,7 +313,7 @@ h3 {
   }
 
   a {
-    color: lighten($ui-base-color, 26%);
+    color: lighten($ui-base-color, 34%);
     text-decoration: underline;
   }
 
@@ -363,7 +363,7 @@ h3 {
 }
 
 .button-primary {
-  background-color: $ui-highlight-color;
+  background-color: darken($ui-highlight-color, 3%);
 }
 
 .text-center {