[Story] Burnup graph : create "release" entity

[ptrouillard]

As a Project manager, I would like to display the burnup of the current release in order to trigger fixing actions before the end of the release

It involves: To have a release entity with following data:

• targeted business value e.g: 12000
• target story points e.g: 1000 story points
• start date
• end date

Note that the "release" entity should not be linked to the "sprint" entity at a first glance (may be later on)

This story should focus on creating the release entity with its CRUD Interface