Add return type annotation to addEmojiInput

Summary:
Flow does not infer generics, so generic functions should have annotated return
types. This will become an error in the next Flow version.

Note that this uncovered a bunch of errors, which I mostly fixed. I suppressed
a few that would require code changes.

Reviewed By: nmote

Differential Revision: D10419507

fbshipit-source-id: 84b8f33f3c8fe272b18e35e838a0ebf7bdbaa6ff

src/component/base/DraftEditorProps.js

@@ -47,6 +47,7 @@ export type DraftEditorProps = {
   // styling and formatting when re-applying styles.
   editorKey?: string,
 
+  // $FlowFixMe in practice people pass other renderable things here
   placeholder?: string,
 
   // Specify whether text alignment should be forced in a direction

src/model/constants/DraftHandleValue.js

@@ -19,4 +19,4 @@
  * A type that allows us to avoid returning boolean values
  * to indicate whether an event was handled or not.
  */
-export type DraftHandleValue = 'handled' | 'not-handled';
+export type DraftHandleValue = 'handled' | 'not-handled' | boolean;