Create iCalendar endpoint for Access Token expiry

To have an overview of expiring Personal|Group|Project Access Token a calendar view of upcoming expiry dates would be helpful.

Exposing the expiry dates in iCalendar format would enable our users to import into their calendar tool of choice. With our path-dependent feed token for .ics endpoints the feed_token in the iCalendar URL will not be usable for any other URL.

Implementation tracking:

• Personal Access Token available at https://gitlab.com/-/user_settings/personal_access_tokens.ics
• Group Access Token - TBD
• Project Access Token - TBD