html: use <base href=...>

Avoid repeating always the same base path.
4 jobs for html_reorg in 5 minutes and 17 seconds (queued for 1 second)
latest
Status Job ID Name Coverage
  Container
passed debian #2793036

00:00:22

 
  Build
passed build #2793037

00:04:55

passed py.test #2793038

00:00:31

passed tox #2793039

00:02:18