Add field for GitHub organizations (to simplify GitHub repo administration) in PMI

In order to reduce maintainance efforts and avoid PMI editing (staff user) errors, I propose the following:

add a GitHub organization field to the Source Code and Issues\Bugzilla section in PMI above the GitHub repositories fields
Similar to the GitLab Project Group field that was added via #92 (closed) it should allow to specify a GitHub organization (if the Eclipse project has it's own dedicated GitHub org, e.g. https://github.com/eclipse-pde)
If the field is used (e.g. set to https://github.com/eclipse-pde), then
- no GitHub repo should need to be specified individually in the GitHub Repositories fields in PMI anymore
- all repositories in this GitHub org should be associated with the eclipse.pde project and listed in the Developer Resources tab
- the GitHub sync script should set the correct permissions for all GitHub repos in the specified GitHub org
for Eclipse projects that do not have a dedicated GitHub org (e.g. projects that have repos in the Eclipse GitHub org), the existing GitHub repositories fields should still work
Bonus: radio selection to be able to either use GitHub organization field or GitHub repositories fields

What would be the benefits?

If new repos are added/removed/renamed in a dedicated GitHub org nothing needs to be changed in PMI (one thing less to remember!)
Not having to specify individual repos avoids potential copy&paste errors
More alignment with the way GitLab repositories are handled in PMI (by only specifying the GitLab Project Group)