Allow subscribing and starring of all Issues and Pull requests

[andrew]

When viewing issues or pull request that you don't have a notification for in Octobox, we can potentially allow the user to star and subscribe to those notifications so they can find them later.

Currently the UI disables the Archive and Delete buttons if no notification is found:

This will require some changes in the octobox code base as well, likely with a new API endpoint that makes a "fake" notification record pointing at the subject that doesn't have a notification ID on GitHub yet, with the potential to merge it with one if it's created later.