Fix incident impact

[rpiazza]

No description provided.

[chisholm]

My observations so far (I haven't gotten through everything yet):

general:

• property "extension_type" should never be directly defined on a custom extension. That's done for you automatically during registration.

If you wanted me to check the python implementation for compliance with the spec as it currently exists in the common objects repo:

identity:

• contact-number.contact_number_type is optional in the spec, required in python
• email-contact.digital_contact_type is optional in the spec, required in python
• social-media-contact.digital_contact_type is optional in the spec, required in python
• in the extension itself, contact_numbers and email_addresses are both defined using ListProperty(EmbeddedObjectProperty(...)), but social_media_accounts is defined as just ListProperty(...). The style is inconsistent (I prefer the latter, and that's how I directed the other devs to implement it).

observed-string:

• observed-string.extension_type doesn't belong. observed-string is an extension SCO; an "extension_type" property needs to be in the extension dictionary, not at the top level.

incident:

• incident.impacted_entity_counts seems no longer to be defined in the spec
• impact.impact_category is optional in the spec, required in python
• impact.sub_impact_refs is defined in spec, not in python
• impact.start_time may be equal to end_time in spec; not allowed in python

[rpiazza]

I merged in this branch on 4/5. Please review the main branch