Fix two issues with unusual milestone workboard initialization orders

Summary: Fixes T10316. Fixes T10311.

Test Plan:

• Create a project, create a milestone, go back to the parent, go to the workboard. Previously, fatal. Now, prompts you to create workboard.
• Create a project, create a milestone, create the parent workboard, put a task in the milestone's column, go to the milestone workboard. Previously, fatal. Now, prompts you to create workboard.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10311, T10316

Differential Revision: https://secure.phabricator.com/D15232