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/sass/components/_modal.scss

@@ -21,7 +21,7 @@
 
         .custom-textarea {
             max-height: 60vh;
-            min-height: 20em;
+            min-height: 8em;
         }
 
         .suggestion-list {