refactor: Extract people.is_valid_email_for_compose().

It's easier to unit test logic inside of people.js than compose.js.

We allow users to compose emails to any of our cross-realm bots.
Someday we may tighten up which cross-realm bots are valid targets,
since it's not necessarily the case that those bots do anything
useful when you send them messages.

frontend_tests/node_tests/people.js

@@ -67,6 +67,7 @@ initialize();
     var active_user_ids = people.get_active_user_ids();
     assert.deepEqual(active_user_ids, [isaac.user_id]);
     assert.equal(people.realm_user_is_active_human_or_bot(isaac.user_id), true);
+    assert(people.is_valid_email_for_compose(isaac.email));
 
     // Now deactivate isaac
     people.deactivate(isaac);
@@ -74,6 +75,7 @@ initialize();
     assert(!person);
     assert.equal(people.get_realm_count(), 0);
     assert.equal(people.realm_user_is_active_human_or_bot(isaac.user_id), false);
+    assert.equal(people.is_valid_email_for_compose(isaac.email), false);
 
     var bot_botson = {
         email: 'botson-bot@example.com',
@@ -654,6 +656,8 @@ initialize();
 
     assert.equal(people.realm_get('alice@example.com').full_name, 'Alice');
     assert(people.is_cross_realm_email('bot@example.com'));
+    assert(people.is_valid_email_for_compose('bot@example.com'));
+    assert(people.is_valid_email_for_compose('alice@example.com'));
     assert(people.is_my_user_id(42));
 
     assert.equal(global.page_params.realm_users, undefined);

static/js/compose.js

@@ -333,16 +333,7 @@ exports.update_email = function (user_id, new_email) {
 
 exports.get_invalid_recipient_emails = function () {
     var private_recipients = util.extract_pm_recipients(compose_state.recipient());
-    var invalid_recipients = [];
-    _.each(private_recipients, function (email) {
-        if (people.realm_get(email) !== undefined) {
-            return;
-        }
-        if (people.is_cross_realm_email(email)) {
-            return;
-        }
-        invalid_recipients.push(email);
-    });
+    var invalid_recipients = _.reject(private_recipients, people.is_valid_email_for_compose);
 
     return invalid_recipients;
 };

static/js/people.js

@@ -525,6 +525,18 @@ exports.small_avatar_url = function (message) {
     return url;
 };
 
+exports.is_valid_email_for_compose = function (email) {
+    if (people.is_cross_realm_email(email)) {
+        return true;
+    }
+
+    var person = people.get_by_email(email);
+    if (!person) {
+        return false;
+    }
+    return active_user_dict.has(person.user_id);
+};
+
 exports.realm_get = function realm_get(email) {
     var person = people.get_by_email(email);
     if (!person) {