Refactor JWT secret generating & decoding code (#29172)

Old code is not consistent for generating & decoding the JWT secrets.

Now, the callers only need to use 2 consistent functions:
NewJwtSecretWithBase64 and DecodeJwtSecretBase64

And remove a non-common function Base64FixedDecode from util.go

routers/install/install.go

@@ -409,7 +409,7 @@ func SubmitInstall(ctx *context.Context) {
 		cfg.Section("server").Key("LFS_START_SERVER").SetValue("true")
 		cfg.Section("lfs").Key("PATH").SetValue(form.LFSRootPath)
 		var lfsJwtSecret string
-		if _, lfsJwtSecret, err = generate.NewJwtSecretBase64(); err != nil {
+		if _, lfsJwtSecret, err = generate.NewJwtSecretWithBase64(); err != nil {
 			ctx.RenderWithErr(ctx.Tr("install.lfs_jwt_secret_failed", err), tplInstall, &form)
 			return
 		}