Add `/mod` commands

[siew24]

Below is a list of subcommands that are to be added into v1.0:

• [x] deadline - Manage deadlines
  • [x] add - Adds a deadline
  • [x] delete - Removes a deadline
  • [x] edit ... - Edits an existing deadline
  • [x] extend - extends a deadline
• [x] kick - Kicks a member
• [x] ban - Bans a member
• [x] verify - Verifies a member
• [x] course
  • [x] add - Adds a course
  • [x] remove - Removes a course
    • [x] semester
  • [x] start - Prompts everyone to enroll courses
  • [x] end - Disenroll everyone from their courses
    • [x] apply
  • [x] admin - Apply to be admin