Show GPU usage of Slurm tasks

https://redmine.teklia.com/issues/7395

Requires backend#1770 (closed)

Tasks on Slurm Ponos agents are not assigned to a specific GPU, since we do not know about the many GPUs that all the computers Slurm manages have. Slurm Ponos agents can still ask the Slurm API for a GPU or not, so the frontend needs to display whether or not any GPU is requested for the task.

For agents in a slurm mode only, instead of using the gpu attribute to show the GPU icon with a name, please use the requires_gpu boolean attribute to show something else: a GPU icon by itself, a GPU icon followed by "with GPU", a tag saying GPU, whatever looks best.