Global default branch setting (#11918)

* Global default branch setting

* add to app.ini example per @silverwind

* update per @lunny

Co-authored-by: John Olheiser <john.olheiser@gmail.com>

routers/repo/repo.go

@@ -134,6 +134,7 @@ func Create(ctx *context.Context) {
 	ctx.Data["readme"] = "Default"
 	ctx.Data["private"] = getRepoPrivate(ctx)
 	ctx.Data["IsForcedPrivate"] = setting.Repository.ForcePrivate
+	ctx.Data["default_branch"] = setting.Repository.DefaultBranch
 
 	ctxUser := checkContextUser(ctx, ctx.QueryInt64("org"))
 	if ctx.Written() {