Don't autoname milestones, but do show the previous milestone name as a hint

Summary: Fixes T10347. In the long run maybe we'll try to guess this better, but for now get rid of the "Milestone X" hardcode and just show what the last one was called.

Test Plan:

• Created the first milestone for a project.
• Created the nth milestone for a project.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10347

Differential Revision: https://secure.phabricator.com/D15262