Disable Registration 禁止注册

[phantomlsh]

Is your feature request related to a problem? Please describe.
In some cases like in the Highschool, the system is required to be placed under restriction. For example, only authorized users can access the system, with pre-defined accounts or SSO.

Describe the solution you'd like
First of all, a config setting could be added to disable registration. (is there one?)
Then, a customized SSO interface may be added to enhance the user control, linking the user identity across several systems.

Describe alternatives you've considered
For the second part, OAuth may also work, but with more complexity.

Additional context
Features like SSO can be configured (enable/disable)

[ZsgsDesign]

Done