hidden email: Record our own email as returned by /register.

With the "hidden emails" feature (aka `email_address_visibility`)
that's now in beta on the server, the user's "email" as used as an
identifier for them in the Zulip API can be an opaque one made up by
the server -- different from `auth.email` aka `delivery_email`.  So we
need to switch our concept of "own email" to use the identifier-email;
this is #3196.

The server tells us this user's identifier-email in the `/register`
response, so one necessary step is to remember it.  Do that.

In principle we should then update this value on EVENT_USER_UPDATE.
But (a) the same is true of several other values here, and more
importantly (b) we already don't handle EVENT_USER_UPDATE at all,
either in our `users` state-subtree or in the various denormalized
spots where we store other users' emails, avatars, etc.  So punt.
Changing emails is pretty uncommon; and in the particular case where
it's happening for all an org's users en masse because hidden-emails
was turned on or off, the only near-term-feasible solution will
probably be to have the server just invalidate clients' event queues.

src/boot/store.js

@@ -104,6 +104,9 @@ const migrations: { [string]: (GlobalState) => GlobalState } = {
       ackedPushToken: a.ackedPushToken !== undefined ? a.ackedPushToken : null,
     })),
   }),
+
+  // $FlowMigrationFudge
+  '7': dropCache,
 };
 
 const reduxPersistConfig: Config = {

src/realm/realmReducer.js

@@ -14,6 +14,7 @@ import {
 const initialState = {
   canCreateStreams: true,
   crossRealmBots: [],
+  email: undefined,
   twentyFourHourTime: false,
   emoji: {},
   filters: [],
@@ -41,6 +42,7 @@ export default (state: RealmState = initialState, action: Action): RealmState =>
         ...state,
         canCreateStreams: action.data.can_create_streams,
         crossRealmBots: action.data.cross_realm_bots,
+        email: action.data.email,
         emoji: convertRealmEmoji(action.data.realm_emoji),
         filters: action.data.realm_filters,
         isAdmin: action.data.is_admin,
@@ -49,6 +51,8 @@ export default (state: RealmState = initialState, action: Action): RealmState =>
       };
     }
 
+    // TODO on EVENT_USER_UPDATE for self: update email, isAdmin, etc.
+
     case EVENT_REALM_FILTERS:
       return {
         ...state,

src/reduxTypes.js

@@ -213,6 +213,7 @@ export type PresenceState = {|
  * @prop emoji
  *
  * About the user:
+ * @prop email
  * @prop twentyFourHourTime
  * @prop canCreateStreams
  * @prop isAdmin
@@ -224,6 +225,7 @@ export type RealmState = {|
   filters: RealmFilter[],
   emoji: RealmEmojiById,
 
+  email: string | void,
   twentyFourHourTime: boolean,
   canCreateStreams: boolean,
   isAdmin: boolean,