Improve Twitch EmoteSource mechanism

[lookeypl]

Various enhancements to Twitch EmoteSource bit in chatbot part

• Add appropriate API to add/remove EmoteSource-s per channel
• Add configuration fields holding which emote sources are enabled
• Hook up API to CLI commands ie. twitch emote enable ffz would enable FFZ emotes
• Expand EmoteSource logic:
  • Check if service is available and if there are emotes to fetch, skip emote source if there are issues
  • Handle potential unconventional situations (error responses from APIs)
  • Add missing BTTV emote source support