This PR adds the initial work to implement a password policy as defined in #1538.
Specifically, this implements the following
* Rate limiting for the login handler
* Implementing the ability for system admins to require a user to reset their password
* Implementing a password policy that requires passwords to be a minimum of 8 characters
* Removes the default password (gophish) for admin users to instead have the password randomly generated when Gophish first starts up
* Adds a password strength meter when choosing a new password
Fixes#1538
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
