Move the default machine type list out from the 0015_machinetype.py migration

[ddabble]

Having it in the migration is both non-standard practice, and makes it practically difficult to reference in other parts of the code, like in tests (hence the # See the `0015_machinetype.py` migration for which MachineTypes are created by default comments).

The list could e.g. be moved to one of the following:

• Static field(s) on the MachineType model
• Fixture file(s) - or, preferably, factory classes - which is the standard practice for default model instances
  • This could be done after Create fixtures or mock factories (for creating default database data) #555 has been completed, or possibly in conjunction with it
    • In that case, the same could be done for default InheritanceGroups and Committees