Meltano contribution slack message improvements

[pnadolny13]

Follow on from #479

Suggested improvements:

1. Formatted as open and closed, not split by issues/PRs
2. Add comment count to quickly know if they've gotten a response or not yet
3. Split each activity to an individual slack message so we can comment on them or put emojis
4. Increase frequency to multiple times a day - this relies on a dedup mechanism to be implemented like Feature: duplicate-proof replication using record hashes in STATE sdk#161. Running the extract from github more frequently should be a problem.

[pnadolny13]

My thoughts on this:

• 1 and 3 are at odds with each other. My opinion would be to do 3.
• For 2 - I'll have to check but I think we need a new issue_comments stream in order to do this one
• 4 - Once a dedup mechanism is available this is a trivial change to increase the schedule frequency and switch to incremental syncs. It would simplify the dbt model too since we dont need to look back -1 day once incremental is working.