Restrict !joinchannel to single entry

[PeterMHammond]

If a user types the !joinchannel command more than once, it will attach multiple listeners and duplicate message responses to the channel

Steps to Reproduce:

1. type the following command into anywhere ChapterVerse is active: !joinchannel
2. type the following command again into anywhere ChapterVerse is active: !joinchannel

Expected Behavior: ChapterVerse should attach a single listener to the channel.

Actual Behavior: ChapterVerse attaches multipel listeners to the channel, duplicating responses to actionable text entries.

Additional Information:

• Screenshots/Videos:
• Logs:
• Workarounds: Restarting ChapterVerse resolves the issue, as initial joining of channels is based on unique file list.