shorten userid columns to 64 chars, ref #9186

the user table only uses 64 chars for the user id, no need to create longer columns for `oc_share.uid_owner` and `oc_privatedata.user`. `oc_share.share_with` might also contain emails, so the foreign key comment there might not be correct but that is a different issue. Should solve #9186 by shortening the resulting index `oc_share.file_target_index` to < 1000 chars.