Changed 'let' to 'const' where it was possible and adjusted imports

bigbluebutton-html5/imports/api/common/server/helpers.js

.../meetings/server/modifiers/removeMeetingFromCollection.js

...tations/server/methods/publishSwitchToNextSlideMessage.js

...ons/server/methods/publishSwitchToPreviousSlideMessage.js

...entations/server/modifiers/addPresentationToCollection.js

...ports/api/presentations/server/modifiers/eventHandlers.js

...ions/server/modifiers/removePresentationFromCollection.js

...ports/api/shapes/server/modifiers/addShapeToCollection.js

...tml5/imports/api/shapes/server/modifiers/eventHandlers.js

...ports/api/slides/server/modifiers/addSlideToCollection.js

...ports/api/users/server/methods/listenOnlyRequestToggle.js

...l5/imports/api/users/server/modifiers/handleLockingMic.js

.../imports/api/users/server/modifiers/requestUserLeaving.js

...on-html5/imports/api/users/server/modifiers/userJoined.js

bigbluebutton-html5/imports/startup/server/RedisPubSub.js