PLT-4535/PLT-4503 Fix inactive users in searches and add option functionality to DB user search (#4413)

* Add options to user database search

* Fix inactive users showing up incorrectly in some user searches

* Read JSON for searchUsers API into anonymous struct

* Move anonymous struct to be a normal struct in model directory and upadte client to use it

* Added clarification comment about slightly odd query condition in search

model/user_search_test.go

@@ -0,0 +1,19 @@
+// Copyright (c) 2016 Mattermost, Inc. All Rights Reserved.
+// See License.txt for license information.
+
+package model
+
+import (
+	"strings"
+	"testing"
+)
+
+func TestUserSearchJson(t *testing.T) {
+	userSearch := UserSearch{Term: NewId(), TeamId: NewId()}
+	json := userSearch.ToJson()
+	ruserSearch := UserSearchFromJson(strings.NewReader(json))
+
+	if userSearch.Term != ruserSearch.Term {
+		t.Fatal("Terms do not match")
+	}
+}