Prevent cloning private issues by unauthorized users

Using a crafted request on bug_report_page.php (modifying the 'm_id'
parameter), any user with REPORTER access or above is able to view any
private issue's details (summary, description, steps to reproduce,
additional information) when cloning. By checking the 'Copy issue notes'
and 'Copy attachments' checkboxes and completing the clone operation,
this data also becomes public (except private notes).

Credits to Mustafa Hasan (strukt) strukt93@gmail.com for the finding.

@atrol noted that the same vulnerability also existed in bug_report.php,
although in this case the information disclosure is limited to notes and
attachments (issue data itself does not become accessible).

Added an access level check, so that the operation now fails with an
Access Denied error in both cases.

Backported from 1fbcd9bca2f2c77cb61624d36ddee4b3802c38ea
Fixes 0024365, CVE-2018-9839