Integrated Maintenance Window fields in Management API and UI (#677)

* Added Maintenance Window properties to API and UI

* extended Management API with Maintenance Window schedule, duration, timezone and nextAt properties
* extended integration tests for the above properties
* extended Management UI with Maintenance Window column in Action History grid, added tooltip for next execution
* general refactoring

Signed-off-by: Bogdan Bondar <Bogdan.Bondar@bosch-si.com>

* fixed Sonar issues

Signed-off-by: Bogdan Bondar <Bogdan.Bondar@bosch-si.com>

* changed the documentation help link for maintenance window

Signed-off-by: Bogdan Bondar <Bogdan.Bondar@bosch-si.com>

* added licence header, first refactoring after partial PR review

Signed-off-by: Bogdan Bondar <Bogdan.Bondar@bosch-si.com>

* changes related to PR review findings

Signed-off-by: Bogdan Bondar <Bogdan.Bondar@bosch-si.com>

* last PR review findings

Signed-off-by: Bogdan Bondar <Bogdan.Bondar@bosch-si.com>

hawkbit-ui/src/main/resources/messages.properties

@@ -251,6 +251,7 @@ tooltip.check.for.mandatory=Check to make Mandatory
 tooltip.artifact.icon=Show Artifact Details
 tooltip.click.to.edit = Click to edit
 tooltip.metadata.icon = Manage Metadata
+tooltip.next.maintenancewindow = next on {0}
 #rollout action
 tooltip.rollout.run = Run
 tooltip.rollout.pause = Pause