Fix Sun's at-mentions, broken by Slack's changes to usernames.
Fix Sun's at-mentions, broken by Slack's changes to usernames.

Summary:
Due to Slack's deprecation of usernames, Sun can no longer @-mention
folks via <@name>. However, @name still works for now if we also pass
the param link_names=true, which this commit does along with removing
the angle brackets.

In the long run, the recommended fix is move to using <@userId> since more
changes are coming and that is the official, robust method to
@-mention a user. But this is the fastest way to get our normal workflow
back for devs in #1s-and-0s-deploys. If Slack's support can be believed,
we have at least a year to fix it properly.

Asana: https://app.asana.com/0/31965416896056/426618122186171

Test Plan:
Manually tested, running the server via
`env SUN_DEBUG=1 SLACK_BOT_TOKEN='contents of K99' npm run monitor`

And then running the following in a separate terminal window:
`curl -d 'team_id=T0001&team_domain=example&channel_id=C090KRE5P&channel_name=bot-testing&user_name=nabil&text=sun: ping&trigger_word=sun:' localhost:8080`

After making the change, I received a notification in #bot-testing,
which the same command did not do before making the change.

Reviewers: benkraft, csilvers, amos, drosile

Reviewed By: benkraft

Differential Revision: https://phabricator.khanacademy.org/D39350

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