Add Cache-Control header for _avatar and /plugin-assets contents for reduce network connection and CPU usage.

src/main/scala/gitbucket/core/controller/AccountController.scala

@@ -149,6 +149,7 @@ trait AccountControllerBase extends AccountManagementControllerBase {
 
   get("/:userName/_avatar"){
     val userName = params("userName")
+    response.setHeader("Cache-Control", "max-age=3600")
     getAccountByUserName(userName).flatMap(_.image).map { image =>
       RawData(FileUtil.getMimeType(image), new java.io.File(getUserUploadDir(userName), image))
     } getOrElse {

src/main/scala/gitbucket/core/servlet/PluginAssetsServlet.scala

@@ -26,6 +26,7 @@ class PluginAssetsServlet extends HttpServlet {
           val bytes = IOUtils.toByteArray(in)
           resp.setContentLength(bytes.length)
           resp.setContentType(FileUtil.getContentType(path, bytes))
+          resp.setHeader("Cache-Control", "max-age=3600")
           resp.getOutputStream.write(bytes)
         } finally {
           in.close()