Add support for per-campaign custom headers.

- Add new `headers[]` column to the campain table.
- Add new headers box to the campaign UI that takes a JSON array of
  custom headers like the headers on the SMTP settings UI.
- Headers are added to e-mails and messenger postback webhooks.
- Add cypress tests.

Closes #514.

schema.sql

@@ -78,6 +78,7 @@ CREATE TABLE campaigns (
     altbody          TEXT NULL,
     content_type     content_type NOT NULL DEFAULT 'richtext',
     send_at          TIMESTAMP WITH TIME ZONE,
+    headers          JSONB NOT NULL DEFAULT '[]',
     status           campaign_status NOT NULL DEFAULT 'draft',
     tags             VARCHAR(100)[],