Cleanup CI (#1017)

Combine badges on readme and move ci git submodule to `ext/`

.gitmodules

@@ -8,5 +8,5 @@
 	path = python/pybind11
 	url = https://github.com/pybind/pybind11.git
 [submodule "ci"]
-	path = ci
+	path = ext/ci
 	url = https://gitlab.com/cscs-ci/arbor-sim/arbor-ci.git

README.md

-[![CI status](https://gitlab.com/cscs-ci/arbor-sim/arbor/badges/master/pipeline.svg)](https://gitlab.com/cscs-ci/arbor-sim/arbor/-/commits/master) [![Gitpod Ready-to-Code](https://img.shields.io/badge/Gitpod-Ready--to--Code-blue?logo=gitpod)](https://gitpod.io/#https://github.com/arbor-sim/arbor) 
+[![CI status](https://gitlab.com/cscs-ci/arbor-sim/arbor/badges/master/pipeline.svg)](https://gitlab.com/cscs-ci/arbor-sim/arbor/-/commits/master) [![Build Status](https://travis-ci.org/arbor-sim/arbor.svg?branch=master)](https://travis-ci.org/arbor-sim/arbor) [![Gitpod Ready-to-Code](https://img.shields.io/badge/Gitpod-Ready--to--Code-blue?logo=gitpod)](https://gitpod.io/#https://github.com/arbor-sim/arbor) 
 
 # Arbor Library
 
-[![Build Status](https://travis-ci.org/arbor-sim/arbor.svg?branch=master)](https://travis-ci.org/arbor-sim/arbor)
-
 Arbor is a library for implementing performance portable network simulations of multi-compartment neuron models.
 
 An installation guide and library documentation are available online at [Read the Docs](http://arbor.readthedocs.io/en/latest/).