HTTP cache rework and enable caching for storage assets (#13569)

This enabled HTTP time-based cache for storage assets, primarily
avatars. I have not observed If-Modified-Since from browsers during
tests but I guess it's good to support regardless.

It introduces a new generic httpcache module that can handle both
time-based and etag-based caching.

Additionally, manifest.json and robots.txt are now also cachable.

main.go

@@ -11,6 +11,7 @@ import (
 	"os"
 	"runtime"
 	"strings"
+	"time"
 
 	"code.gitea.io/gitea/cmd"
 	"code.gitea.io/gitea/modules/log"
@@ -40,6 +41,7 @@ var (
 func init() {
 	setting.AppVer = Version
 	setting.AppBuiltWith = formatBuiltWith()
+	setting.AppStartTime = time.Now().UTC()
 
 	// Grab the original help templates
 	originalAppHelpTemplate = cli.AppHelpTemplate