Add equations fields to catalogue categories, catalogue items & systems

[joelvdavies]

We need to add the following fields:

Catalogue items

• number_of_spares_required
• criticality
• is_flagged

Catalogue categories

• is_flagged

Systems

• is_flagged

The is_flagged values can default to false. The other fields are technically optional - they will only be assigned once the scheduled calculation job runs.

• New fields added
• Migration script(s) added to populate the new defaults
• Mock data updated