Check for banned instances + improved logging

[melroy89]

• Check for banned instances on missing places
• Improve logging
• Extend comment & code docs (please, do this always if you finally understand the method, this will help other devs)

Fixes : 18th issue of #1175

[melroy89]

OK it seems to work as expected now on my instance (kbin.melroy.org).

[BentiGorlich]

The one thing I commented on in the code definitely needs to be changed. Otherwise the changes look good to me, though I think it could be useful to prepend the origin class to the log statements like

The instance is banned, url: {url}

Maybe something like:

[ActivityPubManager::getEntityObject] The instance is banned, url: {url}