Make email obfuscation CSP friendly

Obfuscation makes email addresses harder to harvest by inserting
into them HTML elements which are supposed to be invisible in the
rendered HTML (display: none). The style is specified inline, and
inline styles may be prohibited by content-security-policy HTTP
header, breaking the markup if generated html is served from the
server which sets such header. So move the style to CSS and refer
to it via `class` in HTML code to fix this.

Author: AMDmi3

src/htmldocvisitor.cpp

@@ -402,7 +402,7 @@ void HtmlDocVisitor::visit(DocURL *u)
         {
           p = writeUTF8Char(m_t,p);
         }
-        if (*p) m_t << "<span style=\"display: none;\">.nosp@m.</span>";
+        if (*p) m_t << "<span class=\"obfuscator\">.nosp@m.</span>";
         if (size==5) size=4; else size=5;
       }
     }

templates/html/doxygen.css

@@ -1527,6 +1527,10 @@ span.emoji {
          */
 }
 
+span.obfuscator {
+  display: none;
+}
+
 .PageDocRTL-title div.toc li.level1 {
   margin-left: 0 !important;
   margin-right: 0;