reduxTypes: Mark GlobalState's properties as read-only.

We already treat them as read-only everywhere except for one case,
touched in this commit, where we've had to suppress several
different Flow errors [0]. So, mark them as read-only with
`$ReadOnly` [1].

Treating `GlobalState`'s properties as read-only (except for that
one exceptional case) is likely to be very stable: we treat them as
read-only because the Redux state is a tree of objects that are all
treated as immutable; see the Redux docs [2] and our architecture
doc [3].

[0] See Greg's comments on this at
    zulip#4709 (comment)

[1] https://flow.org/en/docs/types/utilities/#toc-readonly

[2] http://redux.js.org

[3] ./docs/architecture.md

src/boot/store.js

@@ -90,13 +90,11 @@ export const cacheKeys: Array<$Keys<GlobalState>> = [
 function dropCache(state: GlobalState): $Shape<GlobalState> {
   const result: $Shape<GlobalState> = {};
   storeKeys.forEach(key => {
-    // $FlowFixMe[incompatible-indexer]
-    // $FlowFixMe[incompatible-exact]
-    // $FlowFixMe[prop-missing]
-    // $FlowFixMe[incompatible-variance]
-    // $FlowFixMe[incompatible-type-arg]
-    /* $FlowFixMe[incompatible-type]
-       This is well-typed only because it's the same `key` twice. */
+    /* $FlowFixMe[cannot-write]
+       This is well-typed only because it's the same `key` twice. It seems
+       like we should have to suppress an error about not having that
+       guarantee, in addition to the more minor-looking `cannot-write`. Not
+       sure why we don't. */
     result[key] = state[key];
   });
   return result;

src/reduxTypes.js

@@ -329,7 +329,7 @@ export type UsersState = User[];
  * See in particular `discardKeys`, `storeKeys`, and `cacheKeys`, which
  * identify which subtrees are persisted and which are not.
  */
-export type GlobalState = {|
+export type GlobalState = $ReadOnly<{|
   accounts: AccountsState,
   alertWords: AlertWordsState,
   caughtUp: CaughtUpState,
@@ -355,7 +355,7 @@ export type GlobalState = {|
   userGroups: UserGroupsState,
   userStatus: UserStatusState,
   users: UsersState,
-|};
+|}>;
 
 /** A selector returning TResult, with extra parameter TParam. */
 // Seems like this should be OutputSelector... but for whatever reason,