Don't automatically check "digest" when "instant" is checked.

[RokeJulianLockhart]

Problem

As mentioned at https://forum.minetest.net/viewtopic.php?p=438257#p438257:

At https://content.minetest.net/users/RokeJulianLockhart/settings/email/, I cannot disable digests but enable instant notifications. I have no need for digests, because I triage each notification individually in my e-mail client.

As #556 (comment) explains, this isn't actually enabling the digest mail for that issue type:

This is just a quirk of the UI - notifications won't be sent in the digest if they've already been sent "instantly"

However, this causes two issues:

1. Those who want both do not receive both - merely the more specific of the two.
2. Those who want merely instant notifications are unnecessarily duped into believing that they are to be sent two notifications per type.

Solutions

Allow the user to disable "digest"-type notifications for notification types irrespective of whether the user is subscribed to them as "instant"-type.

Alternatives

I haven't thought of any.

[rubenwardy]

This is just a quirk of the UI - notifications won't be sent in the digest if they've already been sent "instantly"

[RokeJulianLockhart]

#556 (comment)

@rubenwardy, I'll re-scope the issue, then. Thanks.