Pholio - make create stories include the mock description, if there is one.

Summary:
This diff accomplishes this task by adding an arbitrary metadata store to PhabricatorObjectHandle. This seemed like it would be "necessary eventually"; for example if / when we decide we want to show images in these stories we'd need to add some more arbitrary data. A point of debate is this technique will yield the _current_ data and not the data at the time the transaction was originally made. I can see this being both desirable and non-desirable.

Otherwise, the best way to do this is to make a new transaction type specifically for create and store exactly what data we think we would need.

(and there's probably many other ways but they require much more work...)

Test Plan: viewed some pholio create stories and yes, they had the description showing.

Reviewers: epriestley

Reviewed By: epriestley

CC: Korvin, aran

Maniphest Tasks: T3685

Differential Revision: https://secure.phabricator.com/D6767