feat: Backlinks (#979)

* feat: backlinks

* feat: add backlinkDocumentId to documents.list

* chore: refactor
fix: create and delete backlink handling

* fix: guard against self links

* feat: basic frontend
fix: race condition

* styling

* test: fix parse ids

* self review

* linting

* feat: Improved link styling

* fix: Increase clickable area at bottom of doc / between references

* perf: global styles are SLOW

server/models/Backlink.js

@@ -0,0 +1,27 @@
+// @flow
+import { DataTypes, sequelize } from '../sequelize';
+
+const Backlink = sequelize.define('backlink', {
+  id: {
+    type: DataTypes.UUID,
+    defaultValue: DataTypes.UUIDV4,
+    primaryKey: true,
+  },
+});
+
+Backlink.associate = models => {
+  Backlink.belongsTo(models.Document, {
+    as: 'document',
+    foreignKey: 'documentId',
+  });
+  Backlink.belongsTo(models.Document, {
+    as: 'reverseDocument',
+    foreignKey: 'reverseDocumentId',
+  });
+  Backlink.belongsTo(models.User, {
+    as: 'user',
+    foreignKey: 'userId',
+  });
+};
+
+export default Backlink;