Add a command to sun to print changelogs for the last 5 deploys
Add a command to sun to print changelogs for the last 5 deploys

Summary:

See https://app.asana.com/0/31965416896056/344186384206728

All of the heavy lifting is in the jenkins job, so this was pretty much
just reusing existing functions.

Test plan:
```
JENKINS_API_TOKEN=`cat .jenkins_api_token` SLACK_BOT_TOKEN=`cat .slack_bot_token` SLACK_VERIFICATION_TOKEN=`cat .slack_verification_token` npm run-script start
```
```
http --verbose --form POST localhost:8080/ channel_name="bot-testing" channel_id="C090KRE5P" user_name=colin user_id=U0ALUCXAS text="sun: history" trigger_word="sun:" token=`cat .slack_verification_token`
```
- verify I see a message in bot-testing
- verify the deploy-history jenkins job is started
- verify the changelog for the last 5 deploy is printed to bot-testing

Auditors: benkraft

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