Much progress:

Made a lot of checks case-insensitive to prevent case difference duplication.
"groups(s)" -> "group(s)"
deployGitlabUsersAndGroups(): Make all group names Gitlab safe in advance. (Var `$ldapGroupsSafe`.)
deployGitlabUsersAndGroups(): Oh damn, lots of other stuff! Very ready to get to work on synchronising group members now.
in_array_i(): New helper function, case-insensitive version of `in_array_i()`.
array_key_exists_i(): New helper function, case-insensitive version of `array_key_exists()`.
generateRandomPassword(): New helper function to generate fairly random passwords for new users, as a user for some reason can't be created without a password. (They'll never need it though!) -- See https://gitlab.com/gitlab-org/gitlab-ce/issues/55426