Allow Issue Security Level to be dynamically set.

Currently, for larger environments where there is a large number of groups, it can be troublesome to configure issue security levels for cases where the issue should be accessible to different teams. As an example, we can consider the following use-case:

1. Issue should be available to be worked by the 1st-level response team. This team decides to escalate it to a 2nd-level response team.
2. Now the 2nd-level response team must also have access to this issue. They assess the situation and must include an additional team that is specialized in certain segments.
3. Now these 3 teams must have access to the request.

These use-cases can be unmanageable for an instance that has a large number of groups in each of these levels. It would be helpful if the Issue Security Level field would work as a Group Picker (multiple groups) custom field type.