log user in after a successful sign up (#4615)

options/locale/locale_en-US.ini

@@ -204,6 +204,7 @@ remember_me = Remember Me
 forgot_password_title= Forgot Password
 forgot_password = Forgot password?
 sign_up_now = Need an account? Register now.
+sign_up_successful = Account was successfully created.
 confirmation_mail_sent_prompt = A new confirmation email has been sent to <b>%s</b>. Please check your inbox within the next %s to complete the registration process.
 reset_password_mail_sent_prompt = A confirmation email has been sent to <b>%s</b>. Please check your inbox within the next %s to complete the password reset process.
 active_your_account = Activate Your Account

routers/user/auth.go

@@ -987,7 +987,8 @@ func SignUpPost(ctx *context.Context, cpt *captcha.Captcha, form auth.RegisterFo
 		return
 	}
 
-	ctx.Redirect(setting.AppSubURL + "/user/login")
+	ctx.Flash.Success(ctx.Tr("auth.sign_up_successful"))
+	handleSignInFull(ctx, u, false, true)
 }
 
 // Activate render activate user page