Adjust Revenue Type for Issues to Make Not Billable

[#1]

Description - Please add Revenue Type options for issues so we can distinguish between issues that are billable and not billable to our client. 

Why is this feature important to you - Some issues, such as risks or bugs, need to have hours logged toward them, but those hours may not always be billable to the client. In order to work around the current behavior, we have to convert issues to tasks and make the task not billable, which defeats the purpose of having them be an issue. Or we have to work the issue, but have the user log their hours onto a nonbillable task. This limits our ability to report on issues accurately and creates extra effort when logging time. 

How would you like the feature to work - Add the option to select Revenue Type on issues, just like it's available for tasks.

Current Behaviour - Only tasks can have Revenue Type adjusted individually, making some billable and not billable. Issues are always billable if the project is billable. I