Fix problem with logout button

The logout auth endpoint was returning no response body and type
application/json which is not valid, this commit changes it to return
plain/text instead which makes it valid.

backend/vendor/github.com/go-pkgz/auth/README.md

@@ -532,7 +532,7 @@ Follow to next steps for configuring on the Apple side:
 
 After completing the previous steps, you can proceed with configuring the Apple auth provider. Here are the parameters for AppleConfig:
 
-- _ClientID_ (**required**) - Service ID identifier which is used for Sign with Apple
+- _ClientID_ (**required**) - Service ID (or App ID) which is used for Sign with Apple
 - _TeamID_ (**required**) - Identifier a developer account (use as prefix for all App ID)
 - _KeyID_ (**required**) - Identifier a generated key for Sign with Apple
 - _ResponseMode_  - Response Mode, please see [documentation](https://developer.apple.com/documentation/sign_in_with_apple/request_an_authorization_to_the_sign_in_with_apple_server?changes=_1_2#4066168) for reference, default is `form_post`
@@ -541,7 +541,7 @@ After completing the previous steps, you can proceed with configuring the Apple
     // apple config parameters
 	appleCfg := provider.AppleConfig{
 		TeamID:   os.Getenv("AEXMPL_APPLE_TID"), // developer account identifier
-		ClientID: os.Getenv("AEXMPL_APPLE_CID"), // service identifier
+		ClientID: os.Getenv("AEXMPL_APPLE_CID"), // Service ID (or App ID)
 		KeyID:    os.Getenv("AEXMPL_APPLE_KEYID"), // private key identifier
 	}
 ```

backend/vendor/github.com/go-pkgz/auth/provider/apple_pubkeys.go

@@ -151,7 +151,7 @@ type appleKeySet struct {
 
 // get return Apple public key with specific KeyID (kid)
 func (aks *appleKeySet) get(kid string) (keys *applePublicKey, err error) {
-	if aks.keys == nil || len(aks.keys) == 0 {
+	if len(aks.keys) == 0 {
 		return nil, fmt.Errorf("failed to get key in appleKeySet, key set is nil or empty")
 	}
 

backend/vendor/github.com/go-pkgz/auth/token/jwt.go

@@ -332,7 +332,7 @@ func (j *Service) Reset(w http.ResponseWriter) {
 		MaxAge: -1, Expires: time.Unix(0, 0), Secure: j.SecureCookies, SameSite: j.SameSite}
 	http.SetCookie(w, &xsrfCookie)
 
-	w.Header().Set("Content-Type", "application/json; charset=utf-8")
+	w.Header().Set("Content-Type", "text/plain; charset=utf-8")
 }
 
 // checkAuds verifies if claims.Audience in the list of allowed by audReader