Mark some other issue categories as not-stale. (#2496)
While we have a generic 'long term' label, it seems redundant for some
issues that are already labeled with something that clearly is
open-ended and not something we should expect to have a bounded
timeline. For example, we want to actively curate a backlog of design
ideas and good first issues for folks to browse and pick up, so we
shouldn't be marking them as inactive after any fixed time frame.
Chandler Carruth