[admin] Conflicting approach in extending DeviceAdmin

[nemesifier]

I noticed we use 2 different approaches in extending DeviceAdmin at the same time:

we create a new class, unregister the admin from openwisp-controller and register the extended class in openwisp-monitoring, but then we also monkeypatch the class we just defined.

openwisp-monitoring/openwisp_monitoring/device/admin.py

Lines 170 to 203 in 8da5b20

	def device_admin_get_inlines(self, request, obj):
	# copy the list to avoid modifying the original data structure
	inlines = list(super(DeviceAdmin, self).get_inlines(request, obj))
	if not obj or obj._state.adding:
	inlines.remove(MetricInline)
	return inlines
	return inlines
	DeviceAdmin.inlines += [CheckInline, MetricInline]
	# This attribute needs to be set for nested inline
	for inline in DeviceAdmin.inlines:
	if not hasattr(inline, 'sortable_options'):
	inline.sortable_options = {'disabled': True}
	DeviceAdmin.get_inlines = device_admin_get_inlines
	DeviceAdmin.Media.js += MetricAdmin.Media.js + (
	'monitoring/js/percircle.js',
	'monitoring/js/alert-settings.js',
	)
	DeviceAdmin.Media.css['all'] += (
	'monitoring/css/percircle.css',
	) + MetricAdmin.Media.css['all']
	DeviceAdmin.list_display.insert(
	DeviceAdmin.list_display.index('config_status'), 'health_status'
	)
	DeviceAdmin.list_select_related += ('monitoring',)
	DeviceAdmin.list_filter.insert(
	0, 'monitoring__status',
	)
	DeviceAdmin.fields.insert(DeviceAdmin.fields.index('last_ip'), 'health_status')
	DeviceAdmin.readonly_fields.append('health_status')

I didn't notice this before, @nepython can you help me to fix this? We should be able to just rewrite all the monkey patching in the class definition above those lines linked.