ABC-90 Add POST /emoji/search and GET /emoji/autocomplete API endpoints (#8125)

* Add POST /emoji/search and GET /emoji/autocomplete API endpoints

* Add constant to be clearer

model/emoji_search.go

@@ -0,0 +1,34 @@
+// Copyright (c) 2018-present Mattermost, Inc. All Rights Reserved.
+// See License.txt for license information.
+
+package model
+
+import (
+	"encoding/json"
+	"io"
+)
+
+type EmojiSearch struct {
+	Term       string `json:"term"`
+	PrefixOnly bool   `json:"prefix_only"`
+}
+
+func (es *EmojiSearch) ToJson() string {
+	b, err := json.Marshal(es)
+	if err != nil {
+		return ""
+	} else {
+		return string(b)
+	}
+}
+
+func EmojiSearchFromJson(data io.Reader) *EmojiSearch {
+	decoder := json.NewDecoder(data)
+	var es EmojiSearch
+	err := decoder.Decode(&es)
+	if err == nil {
+		return &es
+	} else {
+		return nil
+	}
+}