replace static subject with summary of content

DocTor's emails have a static subject "Consensus issues", lets make the subject more useful:

• mention the highest loglevel (WARNING, NOTICE, ...) contained in the mail
• mention the type of problems detected
• mention the amount of problems per type detected

the subject for this email: https://lists.torproject.org/pipermail/tor-consensus-health/2018-February/008491.html

would look something like:

warning: certexpiry (2), hsdir disagreement (1), badexit disagreement (1)