[bennu-core] New Feature: Healthcheck support

  * Healthchecks can be used to determine the state of any system within the application
  * Extended bennu-admin with Healthcheck support
[bennu-scheduler] Log the user running Custom Tasks

  * Closes #88
Adding favicon to configuration. Closes #111
Removed deprecatd AuthenticationListener in favor of the synchronous version
Remove cross application server session invalidation
Removed accessibleGroups() feature
Add checksum to GenericFile
Create the concept of default file storage to handle unconfigured files
Reviewed group mechanics, separated persistent part from the group logic. Reviewed group language, now using antlr4 and solved all parenthesis issues. Improved error handling.
Extended file API to include an isAccessible(User user) method. New file type: GroupBasedFile that implements isAccessible with a Group
Migration instructions