add a replay endpoint to the API

[pulkitsinghal]

Let's say we created a job via a curl request that had a payload (env and stdin etc.) and in response we ended up with JOB_ID=aaa

To take the feature set further, how about adding an endpoint like:
POST /api/jobs/<JOB_ID>/replay
which will automatically create another job and return a new JOB_ID=bbb

The merit is that we don't post the request data again and therefore we don't make a human error when trying to replay something from the past.