Pre-defined Checks

Many bots end up implementing common checks: fetch power levels, compare to a threshold, raise a `CheckError`. There are no built-in check decorators in `checks.py` beyond `cooldown`.

The idea is to ship a set of ready-made check decorators that cover the most common permission patterns and other useful ones.

Seen the sub issues for the checks to implement. If anyone wants to propose one (without necessary working on it), just send it in the comments.