Display changed attachments as other changes in Maniphest

Summary:
When changing attachments, the removed part is squished together
with the added parts, making it hard to read. This changes the output
so it looks like other changes, seperating each action by a semicolon.

Test Plan:
Viewed a task where i had attached and deleted revisions, and
saw the output look as other changes of same type.

Reviewers: epriestley, btrahan

Reviewed By: epriestley

CC: aran, epriestley

Differential Revision: 1238