Refactor DbContext naming

[Foxtrek64]

Across multiple files, the following naming scheme is used:

_context = DatabaseContext
Context = CommandContext

For instance,

Major/MajorInteractiveBot/Modules/GuildModule.cs

Lines 130 to 133 in 622cc09

	guild.CommandPrefix = newPrefix;
	await _context.SaveChangesAsync();
	// await ReplyAsync($"Guild prefix updated: {oldPrefix} -> {guild.CommandPrefix}");
	await Context.AddConfirmation();

This can lead to confusion and is generally pretty terrible naming overall.

This issue will be used to track the places this change needs to take place.