monitoring/lib: Add methods `Service::getHostName()' and `Service::getName()'

The upcoming service command objects will require a service object.
In order to build the command string, the service command objects require
retrieval of the service's host name and the service's name.

The actual implementation of the methods is subject to change

refs #6593
monitoring/commands: Add common `ScheduleDowntimeCommandForm'

`ScheduleDowntimeCommandForm' should be the base class for forms scheduling downtimes.

refs #6593
monitoring/commands: Fix `ToggleFeatureCommandForm::createElements()'

`Icinga\Web\Form::createElements()' no longer requires to return an array of elements
because elements should be created directly.

refs #6593
monitoring/commands: Add `ToggleNotificationsCommandForm'

`ToggleNotificationsCommandForm' is the form for enabling/disabling host and service notifications on an Icinga instance.

refs #6593
monitoring/commands: Add `DisableNotificationsCommandForm'

`DisableNotificationsCommandForm' is the form for disabling host and service notifications w/ an optional expire date and time on an Icinga instance.

refs #6593
monitoring/commands: Fix `CommandForm::getTransport()'s PHPDoc

refs #6593
modules/command: Add command description to `DisableNotificationsCommandForm'

refs #6593
modules/commands: Add common `ScheduleDowntimeCommand'

`ScheduleDowntimeCommand' is the base class for commands scheduling downtimes.

refs #6593
modules/command: Remove old `ScheduleDowntimeCommand'

refs #6593
monitoring/commands: Add command to schedule a service downtime

refs #6593
monitoring/commands: Add command form for scheduling a service downtime

refs #6593
monitoring: Introduce `ServiceController'

In the long term, `ServiceController' and the upcmoing `HostController' should replace `ShowController'.

refs #6593
monitoring/commands: Add common `AddCommentCommand'

refs #6593
monitoring/commands: Let `ScheduleDowntimeCommand' extend `AddCommentCommand'

refs #6593