Slack bot and other email alternatives

[yoavartzi]

Each user must have one push channel active. The default is email. This channel is used to send the weekly digest and potentially posting reminders (maybe combined with the digest to reduce traffic). We can consider alternatives to emails, such as a Slack bot, or other platforms that allow similar integration. This way instead of an email, the user will get a Slack message (or a sequence of messages) with the digest.