Nested projects for grouped releases

Previous issues such as JRA-1630 have suggested allowing projects to be nested. This has been addressed in the "project category" feature.

However, we have a need for projects to have "grouped releases", where we define a single release for a group of projects, and release them all at once.

As it stands, we either have to define these projects as components of an umbrella project, or define our releases for each project separately.

Defining them as components doesn't cut it, since we need to define permissioning for each one (which JIRA can't do), and defining them all as top-level projects entails too much duplication of data and risk of error.

So while nested projects would be preferred, we could also get by with allowing permissioning at the component level.