Rename all code that refers to 'Singularity' as 'Apptainer'

[prioux]

We've got a bunch of code that refers to singularity. We should just rename everything (variables, info text etc) to Apptainer. Also the default command to invoke internally should switch to 'apptainer'.

Most of this is purely esthetic.

The only thing we should keep is the Userfile model 'SingularityFile'.

[MontrealSergiy]

Should model fields/db columns remain same, for instance, should we keep ToolConfig.singularity_overlays_specs or rename into ToolConfig.apptainer_overlays_specs ?
also singularity_use_short_workdir, singularity_executable_name