EmailBackend cannot be changed via configuration variable

[Moist-Cat]

Is your feature request related to a problem? Please describe.
During development is easier to make tests sending the emails to stdout, instead of using a real SMTP server so it would be beneficial to be able to change it.

Describe the solution you'd like
Add a configuration variable called EMAIL_BACKEND which defaults to indico.vendor.django_mail.backends.smtp.EmailBackend.
Is really a small change; I already have it working in my fork.

[ThiefMaster]

Any reason against using maildump as a "real" SMTP server as suggested in the dev setup docs? This is much more convenient to read the emails sent to it than just dumping raw mail content to stdout.

[Moist-Cat]

Yeah, but that would add aditional dependiencies and I would have to read even more docs before starting to make changes which have nothing to do with emails. So I thought it was overkill to use yet another tool when I could just dump them in console for now like I would do with Django.

[ThiefMaster]

It's as simple as pipx install maildump (which installs it in an isolated virtualenv) and starting it :)

Anyway, feel free to send a PR, it's not something I plan to implement but if there's a good PR for it I won't be opposed to merging it.