add layout field to run model [#186530687]

[uraniumanchor]

Contributing to the Donation Tracker

• I've added tests or modified existing tests for the change.
• I've humanly end-to-end tested the change by running an instance of the tracker.

Issue from Pivotal Tracker

https://www.pivotaltracker.com/story/show/186530687

Description of the Change

In the interest of not making the tech_notes field an essay every time, this is the first step towards splitting out some things. The layout field is used for every single run, so it deserves its own field. It won't show up in api requests unless you ask for tech notes and have the correct permission.

Verification Process

Ran it locally, checked that the v1 api returned the field when requested, that you could edit it in the admin, etc. No complicated validation here.