Notify the user in the UI to not use the Catalog in standalone mode

[taban03]

Describe the bug Accessing the API Catalog without Gateway is not supported (as it needs to authenticate via GW), so It is not meant to be used without GW. If the user tries to access the Catalog in standalone mode, he won't be able to use it and he won't get any explanation or message with the action to take.

Steps to Reproduce

1. Try to access Catalog directly on its port (10014)
2. You won't be able to login, or in case you have a valid JWT token, you will be able to access dashboard but not data from the backend will be fetched

Expected behavior

1. If the user gets a token from Gateway and then tries to access standalone Catalog, he should be redirected to some error page (i.e 500), rather than to the Dashboard.
2. If the user tries to perform the login in the standalone Catalog, an error message should be displayed, notifying him that he should not use the Catalog without Gateway.

https://github.com/zowe/api-layer/wiki/Issue-management

[anton-brezina]

#1921 is somewhat related to this.