Add Synology DS218j (#1568)

device-types/Synology/DS218j.yaml

+---
+manufacturer: Synology
+model: DS218j
+slug: synology-ds218j
+part_number: DS218j
+u_height: 0
+weight: 870
+weight_unit: g
+airflow: front-to-rear
+comments: '[Synology DS218j Specs](https://global.synologydownload.com/download/Document/Hardware/DataSheet/DiskStation/18-year/DS218j/ger/Synology_DS218j_Data_Sheet_ger.pdf)'
+is_full_depth: false
+interfaces:
+  - name: LAN 1
+    type: 1000base-t
+power-ports:
+  - name: External PSU 1
+    type: dc-terminal
+    maximum_draw: 17
+    allocated_draw: 7