Add popup to autocomplete commands

client/stylesheets/base.less

client/views/app/messagePopup.coffee

client/views/app/messagePopupConfig.coffee

client/views/app/messagePopupConfig.html

client/views/app/messagePopupSlashCommand.html

packages/rocketchat-lib/lib/slashCommand.coffee

packages/rocketchat-lib/package.js

packages/rocketchat-lib/server/slashCommand.coffee

packages/rocketchat-slashcommands-invite/invite.coffee

packages/rocketchat-slashcommands-invite/package.js