Add announce page

Taken from http://prod.game1.ll.sdo.com/resources/maintenace/maintenance.php

Signed-off-by: Yuan Si <do4suki@gmail.com>

handler/announce.go

@@ -8,6 +8,7 @@ import (
 	"honoka-chan/encrypt"
 	"honoka-chan/resp"
 	"honoka-chan/utils"
+	"html/template"
 	"net/http"
 	"time"
 
@@ -15,7 +16,10 @@ import (
 )
 
 func AnnounceIndexHandler(ctx *gin.Context) {
-	ctx.String(http.StatusOK, "Hello, world!")
+	ctx.HTML(http.StatusOK, "announce.tmpl", gin.H{
+		"title":   "Love Live! 学园偶像祭 非官方服务器",
+		"content": template.HTML("目前开发完毕的功能包括：登录、相册、Live、个人信息。<br>其他功能仍在开发中，有报错属于正常现象。"),
+	})
 }
 
 func AnnounceCheckStateHandler(ctx *gin.Context) {