LibWeb: Make Element attribute getters take a StringView

These functions are deferring to NamedNodeMap::get_attribute which
already takes a StringView. This changes also leads to finding some
places which were passing though a const char* instead of an entry from
Attribute names. Fix that where applicable, and switch to has_attribute
in some of those places instead of deprecated_attribute where
equivalent.

Ideally this should be taking a 'FlyString const&', but to continue
porting away from DeprecatedString, just leave a FIXME for now.

Userland/Libraries/LibWeb/HTML/HTMLHeadingElement.h

@@ -25,7 +25,7 @@ public:
     virtual DeprecatedString aria_level() const override
     {
         // TODO: aria-level = the number in the element's tag name
-        return get_attribute("aria-level");
+        return get_attribute("aria-level"sv);
     }
 
 private: