Add significant staff dates reminders

[eoinkelly]

Extend Today @ Ackama bot to support:

1. Birthday reminders
2. Work anniversary reminders

Storing state

We need somewhere new to store state because:

• ProjectWorks has values for people's start dates but the data does not seem to be reliable.
• We do not currently have any data storage for:
  • Birthdays
  • A preference about whether you want your birthday celebrated at work
  • A preference about whether you want your work anniversary celebrated at work

We chose Contentful because:

• Our needs stay easily within their free plan.
• The people-team have a usable administration UI out of the box that doesn't require a dev to
  help manage.

We created a single Staff Member data type (see below). We tried to keep the
data stored in Contentful as minimal as possible to avoid duplication e.g.
Contentful does not store the user's real name - that value is looked up in
Slack as required because the user's slack profile is most likely to be up to
date.

// Partial schema dump of the 'Staff Member' content type in Contentful. See the
// Contentful admin UI for the most up to date version of this.
{
  "name": "Staff Member",
  "fields": [
    {
      "id": "slackHandle",
      "name": "Slack Handle",
      "type": "Symbol",
      "localized": false,
      "required": true,
      "validations": [
        {
          "unique": true
        }
      ],
      "disabled": false,
      "omitted": false
    },
    {
      "id": "startedOn",
      "name": "Started On",
      "type": "Date",
      "localized": false,
      "required": true,
      "validations": [],
      "disabled": false,
      "omitted": false
    },
    {
      "id": "bornOn",
      "name": "Born On",
      "type": "Date",
      "localized": false,
      "required": true,
      "validations": [],
      "disabled": false,
      "omitted": false
    },
    {
      "id": "birthdayNotificationEnabled",
      "name": "Birthday Notification Enabled",
      "type": "Boolean",
      "localized": false,
      "required": true,
      "validations": [],
      "defaultValue": {
        "en-US": false
      },
      "disabled": false,
      "omitted": false
    },
    {
      "id": "workAnniversaryNotificationEnabled",
      "name": "Work Anniversary Notification Enabled",
      "type": "Boolean",
      "localized": false,
      "required": true,
      "validations": [],
      "defaultValue": {
        "en-US": true
      },
      "disabled": false,
      "omitted": false
    }
  ]
}

[eoinkelly]

I'll update this code to use the structure outlined by PR #42 when it lands.

[eoinkelly]

This change is too specific to Ackama.