Allow configuration of security token for Webhook

Problem/Motivation

For security reasons webhooks should use a security token:

White-listing Webhook connections by IP addresses was deprecated and has been replaced by Webhook security tokens.

We need to add a new text input which allows storing of that security token. Inside the WebHook controller we will then need to check the value of x-memsource-token header and return a 403 response if is is invalid.