Add security bug link to new issue prompt; replace matrix with discourse

[eviljeff]

Prompted by https://mozilla-hub.atlassian.net/browse/AMOENG-2640, where we're making sure to highlight our security bug process, though not strictly a part of it.

Description

As title, adds a link to the bug bounty program for security bugs, and switches out the link to our matrix channel to discourse, which is a better user experience for asynchronous support.

Context

Testing

Checklist

• Add #ISSUENUM at the top of your PR to an existing open issue in the mozilla/addons repository.
• Successfully verified the change locally.
• The change is covered by automated tests, or otherwise indicated why doing so is unnecessary/impossible.
• Add before and after screenshots (Only for changes that impact the UI).
• Add or update relevant docs reflecting the changes made.

[bacharakis]

I think you wanted to write, replace discourse with matrix :D
Sounds great to me! Thank you!