Share more inline "Done" code between Differential and Diffusion

Summary:
Ref T13222. See PHI995. Before making a change to inline rendering, consolidate this code for generating the "alice added inlines comments." and "alice marked X inlines as done." transactions.

Both Differential and Diffusion have four very similar chunks of code. Merge them into shared methods and reduce code duplication across the methods.

(In the next change, I plan to hide the "done" story when the mark affects your own inline, since users marking their own inlines as "done" is generally not very interesting or useful.)

Test Plan: As author and reviewer/auditor, added inlines in Differential and Diffusion. As author, marked own and others inlines as done and undone. Got sensible transaction rendering and persistence of "Done".

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13222

Differential Revision: https://secure.phabricator.com/D19858