Allow to send e-mails using form submission

[rclement]

Currently, only a JSON API endpoint is available at /api/mail. When using SSG without JS AJAX support, sending form with a simple HTTP POST request using the application/x-www-form-urlencoded content-type would be a nice universal and fallback solution.

However, FastAPI does not allow multiple content-type body request for a single route:

• https://github.com/tiangolo/fastapi/issues/990
• https://stackoverflow.com/questions/61872923/supporting-both-form-and-json-encoded-bodys-with-fastapi

The preferred way seems to go towards having separate routes based on content-type.

One more thing: when using a POST form submission, redirections will be required! One easy way to do it would be to setup:

• A success redirect URL as configuration (return a 302 response to this URL)
• An error redirect URL as configuration (return a 302 response to this URL)

This pattern is called "POST/Redirect/GET": https://en.wikipedia.org/wiki/Post/Redirect/Get

As a reference, Formspree is doing something similar with its "Thank You Redirect": https://help.formspree.io/hc/en-us/articles/360012378333--Thank-You-redirect

[rclement]

Some more specifications.

Configuration

Optional new configurations:

• SUCCESS_REDIRECT_URL: redirect to this URL after an e-mail has been successfully submitted using the form submission API
• ERROR_REDIRECT_URL: redirect to this URL if an error occurred when submitting an e-mail using the form submission API

Question: these configurations are required when using the form submission API? Do we redirect to Origin when not specified?

API

• /api/email/form: form submission API with parameters accepted as application/x-www-form-urlencoded content-type