PLT-4404/PLT-4578/PLT-4541/PLT-4542 Replaced third party autosizing textarea with a custom one (#4442)

* PLT-4578 Replaced third party autosizing textarea with a custom one

* Fix Textbox.handleHeightChange not being called

* Removed unused CSS

* PLT-4541 Force EditPostModal to resize upon opening

* Removed usage of jquery from AutosizeTextarea

* Reverted changes made for PLT-4580 as they're no longer needed

webapp/components/edit_post_modal.jsx

@@ -156,6 +156,8 @@ export default class EditPostModal extends React.Component {
 
     onModalShown() {
         this.refs.editbox.focus();
+
+        this.refs.editbox.recalculateSize();
     }
 
     onModalHide() {