Add command to allow invitation to EDGI gcal from chat

[patcon]

Got the working from API explorer, so just need to get it into a hubot command so that we can all share calendar access much easier.

> patcon: @halpy add gcal user xxxxx@gmail.com

# in ruby
rule = Google::Apis::CalendarV3::AclRule.new(
  scope: {
    type: 'scopeType',
    value: 'scopeEmail',
  },
  role: 'role'
)
result = client.insert_acl('primary', rule)
print result.id

calendarId: ekvdm17nis4qd2j4h4djrdfvh0@group.calendar.google.com

{
  "role": "writer",
  "scope": {
    "type": "user",
    "value": "xxxxxx@gmail.com"
  }
}

Here's the lib we'd need, which apparently works similarly to ruby example: https://github.com/google/google-api-nodejs-client

[patcon]

https://developers.google.com/google-apps/calendar/v3/reference/acl/insert#try-it

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