feat: display showcase posts in fortnightly showcase-digest

[michal-skraburski]

This PR closes #126.

Configuration

4 new configuration options were added:

• Pingable role "Showcase Digest Role". (Non-optional)
• Showcase Digest output channel. (Non-optional)
• Showcase Emoji. (Optional, needs identifier, defaults to thumbsup emoji) _1.
• Showcase Lookback (LOOKBACK_N) in Days (Optional, defaults to 14)

_{1. %F0%9F%91%8D is what the thumbsup emoji looks like, this is due to the way we currently grab identifiers}

Showcase triggers

This feature runs a cron job exactly 16:00 UTC+0 every fortnight Friday, as well as permitting only moderators to run a manual digest:
This is achieved statelessly (survives restarts) by using the week index since epoch time, the cron job technically fires every week but it checks whether but this week should fire based on INTERVAL_WEEKS=Math.Round(LOOKBACK_N / 7) plugged into currentWeekIndex() % INTEVERAL_WEEKS. See features/showcase-digest/triggers.ts.

Current state

A showcase digest currently looks like this:

Following this definition of the issue: #126 (comment)