Simplify page tree logic

This is preparation for #6041.

For historic reasons, the code for bulding the section tree and the taxonomies were very much separate.

This works, but makes it hard to extend, maintain, and possibly not so fast as it could be.

This simplification also introduces 3 slightly breaking changes, which I suspect most people will be pleased about. See referenced issues:

This commit also switches the radix tree dependency to a mutable implementation: github.com/armon/go-radix.

Fixes #6154
Fixes #6153
Fixes #6152

common/herrors/errors.go

@@ -50,9 +50,10 @@ func FprintStackTrace(w io.Writer, err error) {
 // Recover is a helper function that can be used to capture panics.
 // Put this at the top of a method/function that crashes in a template:
 //     defer herrors.Recover()
-func Recover() {
+func Recover(args ...interface{}) {
 	if r := recover(); r != nil {
-		fmt.Println("stacktrace from panic: \n" + string(debug.Stack()))
+		args = append(args, "stacktrace from panic: \n"+string(debug.Stack()), "\n")
+		fmt.Println(args...)
 	}
 
 }