eventActions: Mark some more array params read-only, etc.

This is certainly our intent; and it prevents Flow from worrying about
unsound use of mutability, once we tell Flow enough about these action
types that it can know enough to get worried.

Background explanation: Flow needs to know that there aren't two
pieces of code X and Y and a shared mutable value V where (a) X might
mutate V, (b) Y might read V, and (c) the type X sees V as is not a
subtype of (nor the same as) how Y sees V.

When X is a function that Y is calling, and V is an argument Y is
passing in, that's what `$ReadOnlyArray<...>` etc. are for -- they
guarantee X won't mutate V, so (a) doesn't apply and we're safe.
That's what we do in 2 of the 3 spots in this commit.

But here we also have the *return value* of `responseToActions`
playing the role of V -- the function is Y, and its *caller* is X.
Flow worries that the returned data might also be visible somewhere
else that's depending on the data staying specifically `EventAction`,
and the caller could mutate it to be other kinds of `Action`.
Here I'm not so sure how to write down the immutability guarantee,
so instead just call these values `Action`.

Author: gnprice

src/actionCreator.js

@@ -3,7 +3,7 @@ import type { Action, GlobalState, Dispatch } from './types';
 import { clearTypingNotification } from './typing/clearTypingNotification';
 import { EVENT_TYPING_START } from './actionConstants';
 
-export default (dispatch: Dispatch, actions: Action[], state: GlobalState) => {
+export default (dispatch: Dispatch, actions: $ReadOnlyArray<Action>, state: GlobalState) => {
   actions.forEach(action => {
     switch (action.type) {
       case EVENT_TYPING_START:

src/events/eventActions.js

@@ -1,7 +1,7 @@
 /* @flow */
 import { batchActions } from 'redux-batched-actions';
 
-import type { EventAction, Dispatch, GetState, GlobalState } from '../types';
+import type { Action, Dispatch, GetState, GlobalState } from '../types';
 import { pollForEvents } from '../api';
 import { deadQueue } from '../session/sessionActions';
 import eventToAction from './eventToAction';
@@ -10,7 +10,7 @@ import { getAuth } from '../selectors';
 import actionCreator from '../actionCreator';
 import progressiveTimeout from '../utils/progressiveTimeout';
 
-export const responseToActions = (state: GlobalState, response: Object): EventAction[] =>
+export const responseToActions = (state: GlobalState, response: Object): Action[] =>
   response.events
     .map(event => {
       eventMiddleware(state, event);
@@ -29,7 +29,7 @@ export const responseToActions = (state: GlobalState, response: Object): EventAc
       return true;
     });
 
-export const dispatchOrBatch = (dispatch: Dispatch, actions: EventAction[]) => {
+export const dispatchOrBatch = (dispatch: Dispatch, actions: $ReadOnlyArray<Action>) => {
   if (actions.length > 1) {
     dispatch(batchActions(actions));
   } else if (actions.length === 1) {