Load plone.app.discussion code conditionally.

[mauritsvanrees]

This package is optional since Plone 6.1.

[mauritsvanrees]

• I have released this as 3.2.0 and have updated coredev 6.0 and 6.1 to use it.
• Initially I released it as 3.1.5, but I realised it had new features. So I created a 3.1.x branch based on the 3.1.4 release and basically released that one as 3.1.6 for good measure.
• I have removed the master branch. We have a main branch since 2018, but the master branch still lingered. No commits were made on it since then, except one, which was reverted afterwards. I myself initially checked out the master branch yesterday, so let's avoid others doing the same. :-)