Free-form HTTP Header/Body Crafting

[scottmconway]

Similar to android_income_sms_gateway_webhook's approach with UX, I'd like to be able to specify HTTP headers and the HTTP body in a free-form editor.

When sending the request, some headers (eg. content-length) will be overridden.

Explicit protocols such as kodi would still be supported, but their function would be to auto-fill the template, rather than entirely determining how to send the notification (which they do as of now).

Last, there should be a content-type option that will change the quoting style when inserting parameters into the template.

For an MVP, we can say that JSON (or plaintext) is the only permissible content-type, and enforce JSON quoting.