Pending Access is ACE without a User to grant Access to, so move it to the Security Bundle. Adds Events and event handlers for sending emails when a Pending Access record is created. Improves commenting in the codebase. WIP check in as part of #76.

Next is to create similar events upon creating an ACE for an existing User. Suggested implementation detail is to modify the templates used here to add an 'is_pending' flag.