series: List root msg-ids

We are listing msg-ids for patches on their respective pages already.
Let's go a step further and list root msg-id(s) for given series.

root = message that started the series, whether it's the cover letter or
the first patch.

Closes #36

Signed-off-by: Arkadiusz Hiler <arkadiusz.hiler@intel.com>
1 job for master in 1 minute and 31 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Test
passed #1363144
test

00:01:31