[FIX] website_mail: safari mail issue for font awesome

In current safari (tested on 8 and 9) it seems things like:

content: "\f000";

are transformed when getting them javascriptally to:

content: \f000;

which seems to make no sense with CSS specifications but tests verified
this happened.

This broke sending mass mailing in some instance, which is solved by
this commit.

opw-665496

addons/website_mail/static/src/js/website_email_designer.js

@@ -336,7 +336,7 @@
                 _.find(website.editor.fontIcons, function (font) {
                     return _.find(website.editor.getCssSelectors(font.parser), function (css) {
                         if ($font.is('.'+css[3].join(',.'))) {
-                            content = css[1].match(/content:\s*['"](.)['"]/)[1];
+                            content = css[1].match(/content:\s*['"]?(.)['"]?/)[1];
                             return true;
                         }
                     });