Comment "Viewable by" should also include issue security levels

Proposed design:

• Comments use the same issue security scheme as issues within the same project.
• There is a new setting in the issue security scheme that allows you to set the default for comments separately from the default for issues.
• Reporter pertains to the issue, not the comment.
• Visibility via groups and roles can be disabled in favour of this more comprehensive mechanism.

This is to address the following significant concerns:

• Ability to restrict comment visibility to a custom field or reporter. This offers per-user visibility granularity rather than per-system (group) or per-project (role).
• Roles that contain subsets of users could be merged with other roles to ensure visibility to appropriate people. For example if we have people that are in a Tester role and issues show up on Tester dashboards for testing, then we would make a security level that has both Tester and Manager. Right now if they select the Tester role then managers are excluded. Managers aren't part of the testing workflow so they shouldn't have testing issues showing up on their dashboard for example.
• Ability to set default comment visibility.
• Ability to restrict set of comment visibility overrides. It doesn't just show up as some arbitrary list of groups that happen to exist in the system.

Comments are the most significant subdivisions of issues and not being able to control visibility properly is a recurring problem for us.