PLT-4943: Warn when editing a message > 4000 chars (#4789)

* added basic functionality

* css fixes

* css fixes part II: electric boogaloo

* removed css interference with theme changes, for this and post length error

webapp/utils/constants.jsx

@@ -865,6 +865,8 @@ export const Constants = {
     MENTION_SPECIAL: 'mention.special',
     DEFAULT_NOTIFICATION_DURATION: 5000,
     STATUS_INTERVAL: 60000,
+    AUTOCOMPLETE_TIMEOUT: 100,
+    ANIMATION_TIMEOUT: 1000,
     SEARCH_TIMEOUT_MILLISECONDS: 100
 };