implement additional routes for connecting to slack

[kapil1garg]

PR #8 added an initial implementation of connecting to a Slack bot, but there are more routes that probably should be implemented.

Some include:

• sendMessageToSigChannel: announcements SIG-wide instead of just for projects.
• sendMessageToCommitteeChannel: announcements to different committees (e.g., for onboarding committee to assign onboarding mentors)
• sendMessageToPerson: send a person a DM (e.g., to a mentor for things to check in on with students during SIG or mysore)