Feature: Add support for "blocking" users by adding them to a user group

[atc0005]

EZproxy supports a variety of authentication sources, including flat files and LDAP. You are able to use group membership in LDAP (and presumably other sources) to determine final access, either to the system as a whole or to specific resources.

The primary goal is to add specified user accounts to a flat-file (general use case that should work for everyone), but automatically adding user accounts to a LDAP (or Active Directory) group would provide a central mechanism for blocking/unblocking accounts. This could be useful for a variety of reasons.

[atc0005]

Misc research links below. I've only lightly looked into this, need to dig further.

• https://godoc.org/github.com/go-ldap/ldap#ModifyRequest
• https://www.reddit.com/r/golang/comments/cb17s5/add_user_to_a_group_ldap_golang/
• Example of how to add a user to ldap go-ldap/ldap#95
• https://stackoverflow.com/questions/53041048/ldap-cant-add-user-to-group
• https://stackoverflow.com/questions/15205481/how-to-use-go-with-ldap-protocol