[IMPROVE] Start storing Livechat department within rooms (#11733)

packages/rocketchat-livechat/.app/client/lib/hooks.js

...s/rocketchat-livechat/.app/client/views/livechatWindow.js

packages/rocketchat-livechat/.app/client/views/register.js

packages/rocketchat-livechat/.app/imports/client/visitor.js

packages/rocketchat-livechat/server/lib/Livechat.js

packages/rocketchat-livechat/server/lib/QueueMethods.js

...ages/rocketchat-livechat/server/methods/getInitialData.js

packages/rocketchat-livechat/server/methods/registerGuest.js

packages/rocketchat-livechat/server/models/Rooms.js

packages/rocketchat-livechat/server/models/indexes.js