PLT-3143 Added serverside code for custom Emoji (#3311)

* Added model objects for emoji

* Added database tables for emoji

* Added settings for custom emoji

* Added serverside APIs and unit tests for custom emoji

* Added additional validation to catch duplicate emoji names earlier on

* Added additional validation to prevent users from adding emoji as another user

model/emoji_test.go

@@ -0,0 +1,63 @@
+// Copyright (c) 2016 Mattermost, Inc. All Rights Reserved.
+// See License.txt for license information.
+
+package model
+
+import (
+	"strings"
+	"testing"
+)
+
+func TestEmojiIsValid(t *testing.T) {
+	emoji := Emoji{
+		Id:        NewId(),
+		CreateAt:  1234,
+		UpdateAt:  1234,
+		DeleteAt:  0,
+		CreatorId: NewId(),
+		Name:      "name",
+	}
+
+	if err := emoji.IsValid(); err != nil {
+		t.Fatal(err)
+	}
+
+	emoji.Id = "1234"
+	if err := emoji.IsValid(); err == nil {
+		t.Fatal()
+	}
+
+	emoji.Id = NewId()
+	emoji.CreateAt = 0
+	if err := emoji.IsValid(); err == nil {
+		t.Fatal()
+	}
+
+	emoji.CreateAt = 1234
+	emoji.UpdateAt = 0
+	if err := emoji.IsValid(); err == nil {
+		t.Fatal()
+	}
+
+	emoji.UpdateAt = 1234
+	emoji.CreatorId = strings.Repeat("1", 25)
+	if err := emoji.IsValid(); err == nil {
+		t.Fatal()
+	}
+
+	emoji.CreatorId = strings.Repeat("1", 27)
+	if err := emoji.IsValid(); err == nil {
+		t.Fatal()
+	}
+
+	emoji.CreatorId = NewId()
+	emoji.Name = strings.Repeat("1", 65)
+	if err := emoji.IsValid(); err == nil {
+		t.Fatal()
+	}
+
+	emoji.Name = strings.Repeat("1", 64)
+	if err := emoji.IsValid(); err != nil {
+		t.Fatal(err)
+	}
+}