Code cleanup in bug_report.php

- Move init of form variable $f_rel_type to the beginning of the file
- Remove duplicate initialization of form variable $f_master_bug_id
- Use defined $t_project_id instead of $t_master_bug->project_id
- Replace hardcoded numbers with relationship constants
Allow Reporters to Clone issues

Previously, users needed to have update_bug_threshold to be able to
clone issues, since the creation of a relationship with the parent bug
requires to update it.

This patch lets Reporters clone issues, but disables the "Relationship
with the parent issue" field in bug_report_page.php when the user does
not have updater access to the parent bug.

It also modifies the access level check performed against the parent
bug's project in bug_report.php, which is now only needed when a
relationship with it has been specified.

Fixes #11290