Change: move add_users to dedicated users files

[mattmundell]

What

Move add_users out of manage_sql.c.

Why

Reducing size of manage_sql.c. Better per-file organisation of resources.

References

Follows /pull/2709.

Testing

Created group and role with users, looks OK.

o m m '<create_role><name>r test 1</name><users>testuser, m_test</users></create_role>'
o m m '<create_group><name>test 1</name><users>m, m_test</users></create_group>'