Add option to use self-signed SMTP certificates

### 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