[WIP] Slack app

[mgwalker]

Make the bot Slack app compatible, add buttons and stuff

• Standup config (Message buttons: Standup configuration  #123)
• Button to start interview in reminder
• Set OOO instead of doing interview (Message buttons: Allow users to choose between an interview or being marked out of office #120)
• Show report preview before submitting (Message buttons: Create preview message after interview #121)
• Ask users if they are available (Message buttons: Mark as available #122)
• Add /standup or related slash commands
  • /standup create
• Restrict button responses/slash commands to our team

Other goals:

• Ensure the bot can still work as a standalone bot ("custom integration") as well as a Slack app.
• Document how to set the bot up as an app

[codecov-io]

Codecov Report

Merging #126 into develop will decrease coverage by -4.66%.
The diff coverage is 48.68%.

@@            Coverage Diff             @@
##           develop    #126      +/-   ##
==========================================
- Coverage    84.85%   80.2%   -4.66%     
==========================================
  Files           56      65       +9     
  Lines         1321    1450     +129     
  Branches       174     206      +32     
==========================================
+ Hits          1121    1163      +42     
- Misses         200     287      +87

Impacted Files	Coverage Δ
lib/helpers/index.js	100% <ø> (ø)	✅
lib/bot/index.js	100% <ø> (ø)	✅
lib/bot/userReport.js	84% <100%> (+0.66%)	✅
lib/bot/message-builders/standup-config.js	14.81% <14.81%> (ø)
lib/bot/interactive/index.js	15.38% <15.38%> (ø)
lib/bot/interactive/standup-config.js	17.85% <17.85%> (ø)
lib/bot/interactive/reminder-set-ooo.js	28.57% <28.57%> (ø)
lib/bot/interactive/slash-standup.js	33.33% <33.33%> (ø)
lib/helpers/doInterview.js	64.58% <33.33%> (+8.06%)	✅
lib/bot/interactive/reminder-start-interview.js	50% <50%> (ø)
... and 16 more

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