meta(ISSUE_TEMPLATE): Search both open and closed issues

[joshtrichards]

Summary

Change GitHub search link to include not only open, but also closed issues (to catch items already fixed, but not necessarily released yet or backported). Also, change the wording about reporting bugs already noted on the search forum, in order to clarify its okay to report a bug brought up on the Help Forum.

Also:

• Reword "This is a bug" line so that examples that aren't bugs are pointed out first (since "bug" is still misinterpreted to mean "anything not functioning even if it's a config matter" and a quick skimmer will always simply check that box) + added additional hint to utilize the help forum first if unsure
• Minor rewording line about checking against maintained major + latest patch level
• New stipulation line to further encourage as clear/simple reproduction steps as possible (to streamline triaging/resolution)

TODO

• ...

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• Tests (unit, integration, api and/or acceptance) are included
• Screenshots before/after for front-end changes
• Documentation (manuals or wiki) has been updated or is not required
• Backports requested where applicable (ex: critical bugfixes)