Added the ability to block sign ups using a domain blacklist.
What does this MR do?
It adds the ability to block sign ups using a domain blacklist.
As part of this MR, I restructured the Application Settings form to separate Sign up related settings from Sign in related settings and make everything cleaner and easier to read.
Why was this MR needed?
To fight spam.
What are the relevant issue numbers?
Fixes #19749 (closed) Related to #5573 (moved)
Screenshots (if relevant)
