Improve OAuth2 provider (correct Issuer, respect ENABLED) (#34966)

1. Make "Issuer" strictly follow the spec (see comment)
2. Make "/.well-known/openid-configuration" respond 404 if the OAuth2
provider is not enabled.

Then by the way, remove the JSEscape template helper because it is not
needed any more.

templates/swagger/v1_input.json

@@ -1,6 +1,6 @@
 {
   "info": {
-    "version": "{{AppVer | JSEscape}}"
+    "version": "{{.SwaggerAppVer}}"
   },
-  "basePath": "{{AppSubUrl | JSEscape}}/api/v1"
+  "basePath": "{{.SwaggerAppSubUrl}}/api/v1"
 }