NOS: Polymer UI based commit / push / pull / conflict management

Add elements and logic to provide commit / push / pull / conflict management in a polymer-based UI.

Goal is to improve things little by little without having too much work to do every time.

Stage A

• In case a conflict is detected (409 status code is returned) display a message with a link
• When the user clicks the link, take user to branch management screen and if possible open conflict resolution popup automatically (the latter is not done automatically currently)
  => That will not be good enough for general use, but it will be a first step and should be quick to set up
  => Corresponds to option 1

Stage B

• Add element(s) to handle conflict from a polymer UI => make it generic so that we can leverage it later for app definition and whatever we need
• When a conflict is detected, use the new element(s) to handle it
  => Still incomplete but at least we don't need to redirect anymore when a conflict arises
  => Makes simple mode almost usable (only missing the pull action for that)

Stage C

• Add the pull icon and logic => make it generic so that we can leverage it later for app definition and whatever we need
• Improve the status endpoint for that if needed
  => At this point simple mode management is OK

Stage D

• Add the commit / push icons and logic => make it generic so that we can leverage it later for app definition and whatever we need
• Add the possibility to delete WIP
• Improve the status endpoint for that if needed
  => All modes are handled, we're finished
  => Corresponds to option 3