profiles/nexus,cluster/server: Tighten limits

Description of changes

• Introduces per-user memory limits in nexus, like in borg.
• Tightens task limit in borg and nexus to prevent fork bombs from leaving the machine sluggish
  • Current limit does nothing in borg as the OOM killer kicks in first. It's not impossible to recover the machine, but SSH becomes slow as the fork bomb and the OOM killer battle for control (potentially forever).

Maybe consider making this generic in the future, parameterized by the available RAM in the machine (possibly a module?).

Things done

• Tested
• Updated documentation (Wiki/NetBox)
• Breaking change