Fix disconnect allowing DC from VC that user has no access to

[Jackenmen]

Description of the changes

Fixes #6365, (supersedes) fixes #6385
I added a new condition disallowing disconnect when the user:

• is not in a VC that the bot is in,
• cannot connect to that VC,
• and the VC has non-bot users in it (i.e. someone is actually listening to music in there)

Note that this PR might seem larger than it is - the first commit is a formatting-only change (removed else and dedented all lines), see how it looks with whitespaces hidden: cf6055f

The actual changeset is the 2 commits that come afterwards, see diff:
https://github.com/Cog-Creators/Red-DiscordBot/pull/6768/changes/cf6055f92ed84eff62526a7e54a176c71f65043f..456b93e6559e59447c245072566b4208744ee955

Have the changes in this PR been tested?

Yes

[Flame442]

Nothing wrong with your changes (unless the way you refactored caused any of these issues I didn't really look closely), but some feedback on the existing implementation of the checks in this command while I am looking at them.

[Jackenmen]

I've decided to rework the if statements further with some actual documentation in the code comments, which should actually address both of the review comments:

• future readers will no longer be confused because the expected behaviour is clearly stated, meaning I can remove (vote_enabled or (vote_enabled and dj_enabled))
• is_alone is checked, along with can_skip - if either of these is true, the user should be allowed to DC the bot; if not, we have elif conditions that check the specific early-return conditions that we previously had
  • as you might notice, this is a slight change in behaviour - when can_instaskip is True, we do not early return regardless of whether the user can connect to/is in the player's current VC; this is because instaskip is only True if one of the following is true:
    • the user has a DJ role
    • the user is a mod/guild owner/bot owner
    • the player's channel is equal to the user's channel (i.e. the maybe_move_player() function returned True, meaning the player moved to the user's channel)