no-return-promise-mixed-type: Do not allow functions to return promises mixed with something non-promise

# Bad

```
function doStuff(input) {
if (input === "stuff") {
return Promise.resolve(true);
}
return false;
}
```

Consider the following:

```
doStuff("stuff")
.then(result => console.log(result)); // => true

doStuff("other") // *****Cannot call .then on false!*******
.then(result => console.log(result));
```

It should always return a Promise!

# Good

```
function doStuff(input) {
if (input === "doIt") {
return Promise.resolve(true);
}
return Promise.resolve(false);
}
```