Populate cache fields on creation

Some time ago we decided to cache two values in the projects table to avoid continuous lookups when it's not needed.

• has_external_issue_tracker
• has_external_wiki

But I think we forget to populate those fields on creations so we avoid executing updates later on when we're going to use those fields. So in a before_create callback we can set those values to false or the corresponding value instead leave it as nil for be updated later on during a user request.