Convert Notice::asAtomEntry() to use Notice::asActivity() and Activity::asString()

Convert Notice::asAtomEntry() to use Notice::asActivity() and Activity::asString()

We had two ways to generate an activity entry from a notice; one through
Notice::asAtomEntry() and one through Notice::asActivity() and
Activity::asString(). The code paths had already diverged somewhat. I
took the conditions that were in Notice::asAtomEntry() and made sure
they were replicated in the other two functions. Then, I rewrote
Notice::asAtomEntry() to use the other two functions instead.

This change passes the ActivityGenerationTests unit tests, but there
may be some other stuff that's not getting covered.