Added the obm_completeness_assessments table

With this MR I would like to introduce the obm_completeness_assessments where most of the obmgapanalysis results are stored. By now the fields are the output from the settlement layers data extraction, others may come in the future.

Related discussions should be held soon.

\approve @fd @ds

migrations/00007_add-built-area-sources-table.sql

+-- Create the `obm_sources` table
+
+CREATE TABLE public.obm_sources
+(
+    source_id SMALLSERIAL PRIMARY KEY,
+    name VARCHAR(128),
+    uri VARCHAR(256),
+    publication_date TIMESTAMP
+);
+
+ALTER TABLE public.obm_sources
+    OWNER to postgres;