Add an external for livechat received messages

...ages/rocketchat-livechat/client/stylesheets/livechat.less

packages/rocketchat-livechat/client/ui.js

...chat-livechat/client/views/app/tabbar/externalSearch.html

...etchat-livechat/client/views/app/tabbar/externalSearch.js

packages/rocketchat-livechat/config.js

packages/rocketchat-livechat/package.js

packages/rocketchat-livechat/server/externalMessageHook.js

...ketchat-livechat/server/models/LivechatExternalMessage.js

...cketchat-livechat/server/publications/externalMessages.js

packages/rocketchat-theme/assets/stylesheets/base.less