[Fix] Error when trying to show preview of undefined filetype

closes 6922
closes 6917

* Add ESLint rule `prefer-template`

* Add ESLint rule `template-curly-spacing `

* ESLint ignore rocketchat-livechat/assets/rocket-livechat.js

* Add ecmascript package to all packages

* Use ecmascript package for server and client on saml package

* Fix compilation

* ESLint add rule `object-shorthand`

* ESLint rocketchat-livechat/assets/rocket-livechat.js

* Add ecmascript package to all packages

* Use ecmascript package for server and client on saml package

* Fix compilation

* Add eslit rule `one-var`

* ESLint add rule `no-var`

* Add ecmascript package to all packages

* Use ecmascript package for server and client on saml package

* Fix compilation

* focus first textbox element

* Empower esc key.

* Eslint globals.

* added a callback to slashcommands

* if the channel exists the command join acts as open command

* test if the slashcommand has a result callback

* join-command braces fix