Disable promise/always-return rule

[Mrtenz]

When promise/always-return is enabled, you must always return a value from .then(). We sometimes use this for side effects, so I think this rule is too strict.

Blocked by #384.

[Gudahtt]

The idea behind this rule was that most of the time, if you're not awaiting or returning a Promise, it's by accident. We can selectively ignore this rule when it's intentional. It's still worth having a guard-rail against forgetting to await or return a Promise, it's a very easy mistake to make.