feat(schema/bookbrainz): Add tables aiding storage of import data

* Base table for imports storing import ids and types
        + bookbrainz.import
* Tables linking import and relevant data in entitytype_data tables
        + bookbrainz.creator_import
* Tables linking import and relevant data in entitytype_data tables
        + bookbrainz.creator_import
        + bookbrainz.editor_import
        + bookbrainz.publication_import
        + bookbrainz.publisher_import
        + bookbrainz.work_import
* Table to store votes cast to discard an import
        + bookbrainz.discard_votes
* Table to store all origin sources of imported data
        + bookbrainz.origin_import
* Table to store source metadata linked with import and (upon it's subsequent
    upgrade) with entity. The origin_name_id refers to the source of the import,
    a foreign key reference to origin_import. The origin_id is the designated
    id of the import data item at it's source.
    	+ bookbrainz.link_import

schemas/bookbrainz.sql

@@ -610,6 +610,77 @@ CREATE TABLE bookbrainz._editor_entity_visits (
 ALTER TABLE bookbrainz._editor_entity_visits ADD FOREIGN KEY (editor_id) REFERENCES bookbrainz.editor (id);
 ALTER TABLE bookbrainz._editor_entity_visits ADD FOREIGN KEY (bbid) REFERENCES bookbrainz.entity (bbid);
 
+CREATE TABLE IF NOT EXISTS bookbrainz.import (
+    id SERIAL PRIMARY KEY,
+    type bookbrainz.entity_type NOT NULL
+);
+
+CREATE TABLE IF NOT EXISTS bookbrainz.creator_import (
+	import_id INT PRIMARY KEY,
+	data_id INT
+);
+ALTER TABLE bookbrainz.creator_import ADD FOREIGN KEY (import_id) REFERENCES bookbrainz.import (id);
+ALTER TABLE bookbrainz.creator_import ADD FOREIGN KEY (data_id) REFERENCES bookbrainz.creator_data (id);
+
+CREATE TABLE IF NOT EXISTS bookbrainz.edition_import (
+	import_id INT PRIMARY KEY,
+	data_id INT
+);
+ALTER TABLE bookbrainz.edition_import ADD FOREIGN KEY (import_id) REFERENCES bookbrainz.import (id);
+ALTER TABLE bookbrainz.edition_import ADD FOREIGN KEY (data_id) REFERENCES bookbrainz.edition_data (id);
+
+CREATE TABLE IF NOT EXISTS bookbrainz.publication_import (
+	import_id INT PRIMARY KEY,
+	data_id INT
+);
+ALTER TABLE bookbrainz.publication_import ADD FOREIGN KEY (import_id) REFERENCES bookbrainz.import (id);
+ALTER TABLE bookbrainz.publication_import ADD FOREIGN KEY (data_id) REFERENCES bookbrainz.publication_data (id);
+
+CREATE TABLE IF NOT EXISTS bookbrainz.publisher_import (
+	import_id INT PRIMARY KEY,
+	data_id INT
+);
+ALTER TABLE bookbrainz.publisher_import ADD FOREIGN KEY (import_id) REFERENCES bookbrainz.import (id);
+ALTER TABLE bookbrainz.publisher_import ADD FOREIGN KEY (data_id) REFERENCES bookbrainz.publisher_data (id);
+
+CREATE TABLE IF NOT EXISTS bookbrainz.work_import (
+	import_id INT PRIMARY KEY,
+	data_id INT
+);
+ALTER TABLE bookbrainz.work_import ADD FOREIGN KEY (import_id) REFERENCES bookbrainz.import (id);
+ALTER TABLE bookbrainz.work_import ADD FOREIGN KEY (data_id) REFERENCES bookbrainz.work_data (id);
+
+CREATE TABLE IF NOT EXISTS bookbrainz.discard_votes (
+    id SERIAL PRIMARY KEY,
+    import_id INT NOT NULL,
+    editor_id INT NOT NULL,
+    voted_at TIMESTAMP WITHOUT TIME ZONE NOT NULL DEFAULT timezone('UTC'::TEXT, now())
+);
+ALTER TABLE bookbrainz.discard_votes ADD FOREIGN KEY (import_id) REFERENCES bookbrainz.import (id);
+ALTER TABLE bookbrainz.discard_votes ADD FOREIGN KEY (editor_id) REFERENCES bookbrainz.editor (id);
+
+CREATE TABLE IF NOT EXISTS bookbrainz.origin_import (
+    id SERIAL PRIMARY KEY,
+    value TEXT NOT NULL CHECK (value <> '')
+);
+
+CREATE TABLE IF NOT EXISTS bookbrainz.link_import (
+    import_id INT,
+    origin_name_id INT NOT NULL,
+    origin_id TEXT NOT NULL CHECK (origin_id <> ''),
+    imported_at TIMESTAMP WITHOUT TIME ZONE NOT NULL DEFAULT timezone('UTC'::TEXT, now()),
+    last_edited VARCHAR(10),
+    entity_id UUID DEFAULT NULL,
+    metadata jsonb,
+    PRIMARY KEY (
+        origin_name_id,
+        origin_id
+    )
+);
+ALTER TABLE bookbrainz.link_import ADD FOREIGN KEY (entity_id) REFERENCES bookbrainz.entity (bbid);
+ALTER TABLE bookbrainz.link_import ADD FOREIGN KEY (import_id) REFERENCES bookbrainz.import (id);
+ALTER TABLE bookbrainz.link_import ADD FOREIGN KEY (origin_name_id) REFERENCES bookbrainz.origin_import (id);
+
 -- Views --
 
 CREATE VIEW bookbrainz.creator AS