ReviewEvent is_mergeable may not be reliable

[carlosms]

The ReviewEvent.is_mergeable value is taken from the mergeable field of the pull request response. This field may be null while github does the mergeability test, so probably in lookout there are times where we get the event before this is true or false.

From the github docs:

The value of the mergeable attribute can be true, false, or null. If the value is null, then GitHub has started a background job to compute the mergeability. After giving the job time to complete, resubmit the request. When the job finishes, you will see a non-null value for the mergeable attribute in the response.

This issue is to discuss if we want to either remove this field from our ReviewEvent, or make it reliable waiting until github has a definitive value for the field.