[bug 997891] Use failsafe format() to format subject.

Because 'instance' is not yet saved in the db, bug_id is still
unicode and python cannot convert it using '%d'.
Merge pull request #733 from glogiotatidis/997891

[bug 997891] Use failsafe format() to format subject.
[bug 996059] Add waffle flag to settings template.
Merge pull request #734 from akatsoulas/996059

[bug 996059] Add waffle flag to settings template.
[fix bug 996073] Notify mentors and reps about unavailability.
[bug 996059] Restrict access only to Rep group.