Adding global secrets

As a user, I would like to have the option to define sensitive information as global secrets so that I can add them as variables in our templates. The benefits is two-fold:

1) In case the variable value changes, we can do it in just a single place and all usages will be updated automatically

2) We can then decouple the contents of sensitive information and "hide" it from editors. A quick example of this could be an API key that is part of a payload.

In addition, each secret could have an option to be redacted on previews if we still want to prevent the secret from being shown to editors.