Introduce a new Namespace#licensed_feature_available? method

This makes clear that this method is checking for licensed features,
for consistency with `Project#licensed_feature_available?`.
I believe in the end, `Namespace#licensed_feature_available?` should
replace `Namespace#feature_available?` since `Namespace` only checks
for licensed features.

Signed-off-by: Rémy Coutable <remy@rymai.me>