Make "Edit Build Plan" more resiliant so old build configurations can be deleted

Summary: Currently the "Edit Build Plan" page crashes if there are any build steps with invalid implementations (because the implementation class has been removed or renamed). This updates the Edit Build Plan page so that steps with invalid implementations can be deleted.

Test Plan: Looked at a build plan with invalid configurations and deleted it's steps.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley

CC: Korvin, epriestley, aran

Maniphest Tasks: T4111, T1049

Differential Revision: https://secure.phabricator.com/D7708