Add explicit setting for "no email security"

[ianroberts]

The present mechanism for configuring that your email server uses no security is to omit the DJANGO_EMAIL_SECURITY setting from .env altogether. This is somewhat brittle as upgrades and some initial installation scenarios can either leave the variable wrongly set or modify it (from no security to TLS, as that is the default setting for new installs).

This PR allows an explicit

DJANGO_EMAIL_SECURITY=none

to represent an active choice of "no security" in a way that's less likely to be accidentally overwritten.

Closes #401

[github-actions]

Jest Coverage

File	% Stmts	% Branch	% Funcs	% Lines	Uncovered Line #s
All files	83.8	83.96	64	83.8

File	% Stmts	% Branch	% Funcs	% Lines	Uncovered Line #s
All files	83.8	83.96	64	83.8
_jrpc	94.11	91.66	83.33	94.11
_ index.js	94.11	91.66	83.33	94.11	29-30,38-40
_utils	81.97	82.97	57.89	81.97
_ annotations.js	97.72	73.91	100	97.72	35-36
_ dict.js	88.88	83.33	100	88.88	3-4
_ expressions.js	80.08	82.35	80	80.08	...,188-190,201-218
_ index.js	73.6	100	14.28	73.6	...4-65,76-82,93-94