AK+Everywhere: Rename String to DeprecatedString

We have a new, improved string type coming up in AK (OOM aware, no null
state), and while it's going to use UTF-8, the name UTF8String is a
mouthful - so let's free up the String name by renaming the existing
class.
Making the old one have an annoying name will hopefully also help with
quick adoption :^)

Userland/Services/WebServer/Configuration.cpp

@@ -10,7 +10,7 @@ namespace WebServer {
 
 static Configuration* s_configuration = nullptr;
 
-Configuration::Configuration(String root_path)
+Configuration::Configuration(DeprecatedString root_path)
     : m_root_path(move(root_path))
 {
     VERIFY(!s_configuration);