FEATURE: allow email headers to be set via variable

[LvckyAPI]

We have built a contact form for a customer.
The reply-to address should correspond to the address that the website user entered as their own in the form.

Unfortunately, we encountered a problem:

Email "{email}" does not comply with addr-spec of RFC 2822.

With this pull request, I have built in that you can also set the e-mail headers such as: Reply address, sender address, recipient address etc. via variable

[mficzel]

Thanks for the PR I totally understand the motivation.
We decided to not apply the data templates to those fields by default to avoid unneeded security risks.

The changes you suggest can be added in a project easily where one can also better reflect on wether or not they may cause trouble in the concrete use case.

However i admit that this was a very defensive decision and i did not give it that much thought. I wanted to avoid that editors can accidentally create a public email form that can send any email. That is why I made this an opt in feature one has to enable.

If you see a way to address the mentioned risks I am willing to reconsider those decisions.

Regards Martin

[LvckyAPI]

Todo @LvckyAPI

• check mail address validation to pretend to manipulate mail headers
• add warnings if field in inspector contains variable