Add scheduled trigger in scenes

[Pierre-Gilles]

Pull Request check-list

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• If your changes affects code, did your write the tests?
• Are tests passing? (npm test on both front/server)
• Is the linter passing? (npm run eslint on both front/server)
• Did you run prettier? (npm run prettier on both front/server)
• If your changes modify the API (REST or Node.js), did you modify the API documentation? (Documentation is based on comments in code)
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Description of change

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[codecov]

Codecov Report

Merging #673 into master will increase coverage by 0.11%.
The diff coverage is 95.31%.

@@            Coverage Diff             @@
##           master     #673      +/-   ##
==========================================
+ Coverage   91.75%   91.87%   +0.11%     
==========================================
  Files         392      393       +1     
  Lines        4899     4958      +59     
==========================================
+ Hits         4495     4555      +60     
+ Misses        404      403       -1

Flag	Coverage Δ
#server	91.87% <95.31%> (+0.11%)	⬆️

Impacted Files	Coverage Δ
server/seeders/20190227043234-scene.js	100% <ø> (ø)	⬆️
server/utils/constants.js	100% <ø> (ø)	⬆️
server/api/websockets/index.js	33.33% <ø> (+1.12%)	⬆️
server/models/scene.js	93.75% <100%> (+12.5%)	⬆️
server/lib/scene/scene.triggers.js	87.5% <100%> (+1.78%)	⬆️
server/lib/scene/index.js	100% <100%> (ø)	⬆️
server/lib/scene/scene.cancelTriggers.js	100% <100%> (ø)
server/lib/scene/scene.destroy.js	100% <100%> (ø)	⬆️
server/lib/scene/scene.addScene.js	94% <93.75%> (-6%)	⬇️

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