Allow excempting cleanup of starred entries

[treetip]

I just realized I had lost starred items that were older than a year, and checking FeedEntryDAO in master branch, there doesn't seem to be any conditions other than age of the entry.

Describe the solution you'd like
Allow me to keep starred entries infinitely, or for a configurable duration.

Additional context
I personally use stars like bookmarks to store information I want to look up later for reference.