Removed Reddit -> Slack integration and updated handbook

Why is this change being made?

After a conversation with Peter Kaldis, we decided it would be best to remove the Zapier integration that posts new Reddit mentions to the #reddit slack channel.

The reasons are:

1. It is not used except to check if the zap is working
2. It takes up a lot of our allotted Zapier zap count. The Zap quota is counted per each step (not per zap), so zaps that have multiple steps take up a large amount of the quota. The Reddit->Slack integration alone had 7 steps
3. Reddit is one of our highest volume channels, so even with the Zap simplified it would still take up a large amount of the quota

The updated zap can be seen in the community relations folder and the deprecated one was moved to the backups/testing folder. Once the handbook is updated, the #reddit slack channel will be archived.

The reddit handbook update includes a mention of what to do if it seems the zap is broken. If we find out that this isn't working then we can re-evaluate our approach. The #reddit slack channel can always be unarchived.

cc @slee24 @LindsayOlson @annie_huang @borivoje @bvanderkolk

Author Checklist

• Provided a concise title for the MR
• Added a description to this MR explaining the reasons for the proposed change
• Assign this change to the correct DRI