Allow users to be CC'ed on deploy messages
Allow users to be CC'ed on deploy messages

Summary:
See https://app.asana.com/0/31965416896056/359776542661947

This modifies the `deploy` and `deploy-not-risky` commands to look for
an optional CC list of users. These users will be at-mentioned in every
reply from Sun until the list is cleared (the CC list is cleared on
deploy, finish, and abort). Examples:

```
sun: deploy drosile, cc @slacker-cow
sun: deploy drosile, cc: slackbot, @sun-wukong
sun: deploy drosile, cc @channel # <- probably don't do this
```

This commit also adds a `cc` command to sun:

```
sun: cc @slacker-cow

=> "@drosile (cc @slacker-cow) Okay, I've added them to the notification
list for this deploy.

sun: cc: slackbot, @sun-wukong

=> @drosile (cc @slacker-cow, @slackbot, @sun-wukong) Okay, I've added
them to the notification list for this deploy.
```

Note that the `cc` command will only work if a deploy is ongoing.

Test Plan:
- run bot
```
SUN_DEBUG=1 JENKINS_API_TOKEN=`cat .jenkins_api_token` SLACK_BOT_TOKEN=`cat
ck_bot_token` SLACK_VERIFICATION_TOKEN=`cat
ck_verification_token` npm run start
```

- issue cc command
```
curl -d
nnel_name=bot-testing&channel_id=C090KRE5P&user_name=drosile&user_id=U5L7M7YS1&text=sun:
cc drosile&trigger_word=sun:&token=`cat .slack_verification_token`"
lhost:8080
```

- verify I see a message in bot-testing
- verify the message properly shows a CC list

Reviewers: nabil, csilvers

Reviewed By: nabil, csilvers

Subscribers: james, damianne

Differential Revision: https://phabricator.khanacademy.org/D36449

Khan/slack-deploy-hooks,Khan/slack-deploy-hooks