Load the campaign panel asynchronously

[mgax]

To load the campaign panel, if newsletter_campaign is not blank (i.e. it contains the ID of a campaign), we must make API calls to fetch the campaign information. These might well take a few seconds and delay the page load, even if the user is not interested in the newsletter panel, and only e.g. wants to change the page body.

We should load the panel contents asynchronously. Some relevant code from a previous spike is here.