Make it easy for Brigades to opt out of civic json

[ondrae]

In the case that a Brigade doesn't want to participate, we need to respect that.
We could:

• Have an opt out field in the spreadsheet
• That would make a new api attribute under /projects like civic_json_opt_out: True or whatever.

Task Order

• Add the opt_out column to the test spreadsheet
• Write tests for the CfAPI to pull in the opt_out attribute
• Have the CfAPI pull in that attribute
• Merge all that
• Add the column to the real Brigade Info sheet
• Have the Project Search not show the button if they've opted out