More flexible issue comment permissions

There are areas of our business where we like to keep issues public as much as possible. However, situations arise within the public issues where we would like to keep certain comments private between the reporter and us.

Currently, the only access control for comments is based on groups, and only one group can be specified. We would like to see more flexible access control for comments:

• a "reporter" option on visibility
• should be able to specify visibility to multiple groups
• perhaps comment visibility options could be the same as issue security options.