Update Interview command and runner

[zpatrick]

Updates commands:

• Checks that the candidate exists in !interview add
• Removes a candidate's interviews in !candidate rm

Updates runners:

• Create a single CleanupRunner to do cleanup tasks (e.g. old interviews)
• Create a single ReminderRunner to do reminders (e.g. interview reminders)

Other stuff:

• Switch back to nlopes/slack from quintilesims/slack