Simplify the logic of which post gets picked/announced by Blaggregator

[punchagan]

It should be simple for an RCer to figure out whether a post will be Zulip announced or not. Posts unexpectedly (for RCers who haven't seen the code) show up or go missing in the blogging stream.

• The first step could be to document it clearly, and put it in a very visible place.
• See if the logic can be simplified covering most of the use-cases.
• Have a way for users to ask? a form submission or the like?