This implements the first pass for a user management API allowing users with the `ModifySystem` permission to create, modify, and delete users. In addition to this, any user is able to use the API to view or modify their own account information.
* Refactoring servers to support custom workers and graceful shutdown.
* Refactoring workers to support custom mailers.
* Refactoring mailer to be an interface, with proper instances instead of a single global instance
* Cleaning up a few things. Locking maillogs for campaigns set to launch immediately to prevent a race condition.
* Cleaning up API middleware to be simpler
* Moving template parameters to separate struct
* Changed LoadConfig to return config object
* Cleaned up some error handling, removing uninitialized global error in models package
* Changed static file serving to use the unindexed package