GitLab

Resolves #1.

This is made possible by droping the `with_plugins` helper
in rework which makes sub-command override impossible.

Still wondering how it did work 'till now :o

We just show something rather than crashing.

When the tasks list grows long, it becomes annoying to find the
workers/services sections.

This is an heavy backport wich carries a bit too much baggage ...

Noteworthy:
* we add a db table to store tasks table views per domain (and complete)
* the client checks a hash rather than reloading the whole table: only on changes
  does the reload happen
* a domain selector is introduced
* a dedicated url for task errors is provided
* we provide an extension point `tasks_actions` to allow specification
  of callbacks from unknown tiers

* job_input
* job_results
* relaunch-task

Some js parts are needed for this to work.

We have to work around the broken flask blueprint system.