Adapt DB layout to multiple authors and contributors

create_db.sql

@@ -30,7 +30,6 @@ CREATE TABLE articles (
     summary TEXT NOT NULL,
     published BOOL NOT NULL,
     rejected BOOL NOT NULL,
-    author_id       INT             NOT NULL,
     issue_id INT NOT NULL,
     edited_id INT,
     is_in_issue BOOL NOT NULL,
@@ -46,6 +45,22 @@ CREATE TABLE tags (
     PRIMARY KEY (id)
 );
 
+CREATE TABLE articles_authors (
+    article_id INT,
+    author_id INT,
+    PRIMARY KEY (article_id, author_id),
+    FOREIGN KEY (article_id) REFERENCES articles (id),
+    FOREIGN KEY (author_id) REFERENCES users (id)
+);
+
+CREATE TABLE articles_contributors (
+    article_id INT,
+    contributor_id INT,
+    PRIMARY KEY (article_id, contributor_id),
+    FOREIGN KEY (article_id) REFERENCES articles (id),
+    FOREIGN KEY (contributor_id) REFERENCES users (id)
+);
+
 CREATE TABLE articles_tags (
     article_id INT,
     tag_id INT,