LibWeb: Port HTMLInputElement from DeprecatedString to String

And dealing with the fallout of doing so. I am not 100% sure that it is
safe for us to be treating Strings in the value sanitization algorithm
in all cases as if they are ASCII, but this commit does not change any
existing behaviour there.

Userland/Libraries/LibWeb/HTML/FormAssociatedElement.h

@@ -6,7 +6,7 @@
 
 #pragma once
 
-#include <AK/DeprecatedString.h>
+#include <AK/String.h>
 #include <AK/WeakPtr.h>
 #include <LibWeb/Forward.h>
 
@@ -70,7 +70,7 @@ public:
     // https://html.spec.whatwg.org/multipage/forms.html#concept-submit-button
     virtual bool is_submit_button() const { return false; }
 
-    virtual DeprecatedString value() const { return DeprecatedString::empty(); }
+    virtual String value() const { return String {}; }
 
     virtual HTMLElement& form_associated_element_to_html_element() = 0;