Separate pull request info endpoint

[sbinlondon]

Per this message thread: https://codesandboxapp.slack.com/archives/CHM6FA0TA/p1619782410183900?thread_ts=1619780139.183000&cid=CHM6FA0TA

• make pullInfo a separate endpoint in the importer, so we call /pullInfo/:username/:repo/pull/:pullNumber instead of /info/:username/:repo/pull/:pullNumber and we'll prevent conflicts with the import sandbox endpoint

Additionally, we do want to be able to import a specific PR's files as a sandbox, so work should be done on the server afterwards to make sure this is happening.