Make the new Build Plan "Runnable" behavior work

Summary:
Ref T13258. Fixes T11415. This makes "Runnable" actually do something:

• With "Runnable" set to "If Editable" (default): to manually run, pause, resume, abort, or restart a build, you must normally be able to edit the associated build plan.
• If you toggle "Runnable" to "If Viewable", anyone who can view the build plan may take these actions.

This is pretty straightforward since T9614 already got us pretty close to this ruleset a while ago.

Test Plan:

• Created a Build Plan, set "Can Edit" to just me, toggled "Runnable" to "If Viewable"/"If Editable", tried to take actions as another user.
• With "If Editable", unable to run, pause, resume, abort, or restart as another user.
• With "If Viewable", those actions work.

Reviewers: amckinley

Reviewed By: amckinley

Subscribers: PHID-OPKG-gm6ozazyms6q6i22gyam

Maniphest Tasks: T13258, T11415

Differential Revision: https://secure.phabricator.com/D20229