Change Clever Cloud flavours

The XS flavour is just too slow, often failing to run the jobs within reasonable time. Instead, run most jobs under the M flavour, which has two CPUs and more memory. For fast feedback during development, run the “Current” jobs under the L flavour with its four cores.

As a potential offset for the extra requirements, don't retry jobs automatically.

Minor, infrastructure-related jobs that just push stuff remain under XS.

(One of the tests still seemed to fail, so I've marked it as slow.)