- 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.
- Refactor codeflask HTML editor into a standalone html-editor
component.
- Replace the plaintext box in the template editor with html-editor.
- Replace codeflask in the campaign editor with the new html-editor.
- Refactor templates Cypress tests to test the new editor.
- Refactor campaigns Cypress tests to test the new editor and also
test switching between different editors and content formats.
