Add option to use self-signed SMTP certificates

[ahoneybun]

Problem

Currently there is no way that I see to use your own SMTP cert. This is needed for ProtonMail Bridge to work.

Proposal

Enabled a toggle on the IMAP config page (where we enter server, port and other settings) to choice to use your own SMTP cert.

Prior Art (Optional)

This is how Thunderbird which is the only official way of using ProtonMail Bridge on Linux:

https://protonmail.com/bridge/thunderbird#5