Improve the custom field context create/edit UI to allow for easier management of projects and issue types for the custom field

At the moment, in a system with many project and/or issue types it is quite difficult to select propjects and issue types for a custom field context. It is more difficult to change the projects and issue types for a custom field projects.

Moving away from multi-select boxes and replacing them with something like the group joiner/leaver, but with projects instead of groups, would help.