Support Slate instances without anonymous contributions

[SReichelt]

Currently, if the email configuration for anonymous contributions is missing on the server side, the frontend does not know about this and only responds with a "forbidden" error message when the user actually tries to submit a change. This should be improved in order to better support instances without anonymous contributions:

• If neither anonymous contributions nor GitHub integration are enabled, the "edit" button should simply be invisible, except in tutorial mode or when running locally.
• If only anonymous contributions are disabled, it is basically sufficient to change the message "You are currently contributing anonymously" into an error message. Moreover the "Submit" button should be disabled.