feat(ui): type the namespace when the cluster does not list them (#84)

Some users cannot list namespaces on a cluster. The add wizard showed
an error and offered no way forward.

The namespace step now switches to a text prompt when the list call
fails or returns nothing. The prompt shows the reason and checks that
the name is a valid DNS label. Esc from a later step returns to the
prompt.

The j and k keys now type into text fields. They moved the list cursor
before, so a name such as kube-system could not be typed.

Closes #66

internal/config/validator.go

@@ -1,6 +1,7 @@
 package config
 
 import (
+	"errors"
 	"fmt"
 	"regexp"
 	"strings"
@@ -697,6 +698,14 @@ func validateDNS1123Label(name, field, entityType string) *ValidationError {
 	return nil
 }
 
+// ValidateNamespaceName validates a Kubernetes namespace name (a DNS-1123 label).
+func ValidateNamespaceName(name string) error {
+	if verr := validateDNS1123Label(name, "namespace", "Namespace"); verr != nil {
+		return errors.New(verr.Message)
+	}
+	return nil
+}
+
 // validateDNS1123Subdomain validates that a name is a valid DNS subdomain name (RFC 1123).
 // Used for resource names which can contain dots.
 func validateDNS1123Subdomain(name, field, entityType string) *ValidationError {