LDAP: Improve username case senitivity support

[jamoore]

OMERO is currently quite strict with regard to LDAP username enforcement:

    /**
     * Mapping a username to an {@link Experimenter}. This handles checking the
     * username for case exactness. This should be done at the LDAP level, but
     * Apache DS (the testing framework used) does not yet support :caseExactMatch:.
     *
     * When it does, the check here can be removed.
     *
     * @param username
     * @param mapper
     * @return a non null Experimenter.
     * @see ticket:2557
     */
    private Experimenter mapUserName(String username, PersonContextMapper mapper) {
        Filter filter = config.usernameFilter(username);
        List<Experimenter> p = ldap.search("", filter.encode(), mapper);

        if (p.size() == 1 && p.get(0) != null) {
            Experimenter e = p.get(0);
            if (e.getOmeName().equals(username)) {
                return p.get(0);
            }
        }
        throw new ApiUsageException(
                    "Cannot find unique DistinguishedName: found=" + p.size());

    }

There might should be a flag to optionally allow users to "mis-capitalize" their names. However, then there will need to be a case-insensitive UNIQUE constraint on the experimenter.omeName table.

See: ​http://lists.openmicroscopy.org.uk/pipermail/ome-users/2011-March/002587.html