initialDataTypes: Add RawInitialData and start populating.

Greg points out [1] that it's time we start keeping track of a type
for our initial data pre-transformation, since we're about to start
transforming it.

Here, start very small by saying what we expect `realm_users` and
two of its friends to look like. We grab those expectations from the
`avatar_url` field on `User` and on `CrossRealmBot`.

[1] #4230 (comment)

src/api/initialDataTypes.js

@@ -100,6 +100,14 @@ export type InitialDataRealmFilters = {|
   realm_filters: RealmFilter[],
 |};
 
+// TODO: Make complete and exact.
+export type RawInitialDataRealmUser = {
+  realm_users: Array<{| ...User, avatar_url: string | null |}>,
+  realm_non_active_users: Array<{| ...User, avatar_url: string | null |}>,
+  cross_realm_bots: Array<{| ...CrossRealmBot, avatar_url: string | void | null |}>,
+  ...
+};
+
 export type InitialDataRealmUser = {|
   avatar_source: 'G',
   avatar_url: string | null,
@@ -280,7 +288,17 @@ export type InitialDataUserStatus = {|
   user_status?: UserStatusMapObject,
 |};
 
-// Initial data snapshot sent in response to a `/register` request.
+// Initial data snapshot sent in response to a `/register` request,
+// before validation and transformation.
+//
+// TODO: Make complete and exact.
+export type RawInitialData = {
+  ...RawInitialDataRealmUser,
+  ...
+};
+
+// Initial data snapshot sent in response to a `/register` request,
+// after validation and transformation.
 export type InitialData = {|
   // The server sends different subsets of the full available data,
   // depending on what event types the client subscribes to with the