Allow detailed notifications customization

[cheeseblubber]

As part of onboarding let you notify everything when someone joins a community.

Different events:

• Notify everything
• Notify joining a channel
• Notify message in channel you are in
• Notify on DMs
• Notify on @mentions

Include building backend out

Email notification

[sbutler-gh]

The pattern I'm familiar with is you get to set the interval of notifications — which seems to serve a variety of preferences. For example, I could choose to receive email notifications:

• Whenever a notification comes in
• Every 6 hours (email shows all notifications from past 6 hours, if any)
• Every 12 hours (email shows all notifications from past 12 hours, if any)
• Every 24 hours (email shows all notifications from past 24 hours, if any)

[emilos]

We should allow users to set notification per channel too, as per user feedback.