Disallow closed support topics to be modlook reported after 6 months

[netweb]

Via ​https://wordpress.slack.com/archives/C02RQC6RW/p1548897565328900

Currently closed topics can be modlook reported for any time period.

If a topic has been closed for 3 years it should not be able to be modlook flagged.

A valid use case of where a closed topic should be allowed to be reported is:

If a topic was closed yesterday, and a user participating in the topic would like a second opinion on why the topic was closed from a moderator

It was decided in #forums on Slack to use the same time used for auto-closing topics, 6 months.

[coffee2code]

In 8167:

Support Forums: Disallow closed support topics from being 'modlook' reported after 6 months.

Props netweb.
Fixes #4132.