Add option for administrators to approve `user sign-up`

Problem to solve

Add option for administrators to approve user sign-up.

Intended users

• Sidney (Systems Administrator)

Further details

When user sign-up was enabled, a sign-up must be confirmed by an administrator.

Proposal

If user sign-up was disabled, administrators must collect the user information to create users.
If user sign-up was enabled, administrators has no idea about user creation.
Administrators need the privileges to control the user creation but they don't want to collect user information by hand. Therefore, it's better to add an feature that users can input their own information and administrator can approve/reject the user creation.