Add initial logic for building "new" pull requests

This change is very rough around the edges, but it adds initial support
for building "new" pull requests. So far it only tracks whether a pull
request has been seen, and doesn't track the status of the pull requests
build. So even if a pull requests failed to build and was "fixed up", it
won't be automatically run through testing again.

Signed-off-by: Prakash Surya <surya1@llnl.gov>