Add http support

[jfwiebe]

The plugin does not support pretix instances websites running on http. This is not recommended for production environments, but should be implemented for testing purposes.

See the pretix docs for more information on this.

[raphaelm]

Just to be clear: It is not about HTTPS on the pretix instance but on the embedding page :)

[jfwiebe]

Of course, I corrected that. Thanks!