[rndactivity] Write rndactivity cog

[calebj]

Based on @Twentysix26's rndstatus cog.

Features checklist:

• Automatically transform "playing ...", "listening to ...", "watching ..." to appropriate Activity type
  • Parse "streaming [game name](stream URL)" and throw an error if not a Twitch URL
• Variance in delay (will wait delay + randrange(-variance, variance + 1) to switch)
• Better commands/UI:
  • add and remove a list of activities
  • manually set an activity (with optional timer to hold it there)
  • manually skip to next random activity
  • one activity per line in list command
  • detect duplicate activities when adding
• Import from rndstatus
• Suspend when rndstatus is loaded (or override it somehow?)
• Template variables for bot stats: server, member count, voice clients, etc.

Stretch goals:

• entry weights
• custom slots for non-owners to customize
  • owner approves users and role(s) to manage custom slots
  • arbitrary number of slots per user or role
• activity sequences (with same type)
  • short switch time between entries
  • loop until the next status
  • treated as one entry

[calebj]

This'll be v3 instead.