[WIP] Romain changes on monitoring app

Romain prototype with a new functionnality on the monitor app, to allow displaying custom renderjs gadget provided by the software release.

It was releated to this MR: https://lab.nexedi.com/nexedi/slapos/merge_requests/1300#note_174683

It may contain some non related changes that where already in Alain instance:

• graph fix
• some fixes to do in slapos.tolbox to fix graph data