Refactor the `Help` command.

- `redirect_output` has been adjusted to run the `delete_invocation` inside a task as the help command will wait for that to run before sending the help or doing anything else.
- `pagination` has been adjusted to support deleting the paginated message if `cleanup` is True, and an optional `description` that is present through all pages of pagination.

- The help command has been refactored to subclass `commands.HelpCommand`. This means that it now supports methods such as `ctx.send_help(ctx.command)`.
- `help_cleanup` provides the opportunity to use the :x: reaction to cleanup help even with no pagination.
- Pagination purely happens through the `LinePaginator`, forcing a pagination session with 1 line per page where we format the page style before sending it through.
- Categories are properly dealt with by finding a match and sending a seperate help where a named tuple of the Category name, description and relevant cogs is the only parameter.
- Choices for when a command was not found has been updated to include category names, cog names, aliases of group and command names, and include all subcommands and aliases. This should provide a more helpful output when an error message is sent
- Sending command, group, cog, category and bot help has been split into different functions that are called from `command_callback`. This provides an easier way to alter future changes, and cleans up code considerably.

- Important note: no outward facing formatting should have changed. Any desired changes can be discussed in review.

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