(core) add per-user per-org preferences to database

Summary:
Adds preferences to orgs.  There are a few flavors:
 * `userOrgPrefs`: these are specific to a certain user and a certain org.
 * `orgPrefs`: these are specific to a certain org, and apply to all users.
 * `userPrefs`: these are specific to a certain user, and apply to all orgs.

The three flavors of prefs are reported by `GET` for an org, and can be modified by `PATCH` for an org.  The user needs to have UPDATE rights to change `orgPrefs`, but can change `userOrgPrefs` and `userPrefs` without that right since the settings only affect themselves.

Test Plan: added tests

Reviewers: dsagal

Reviewed By: dsagal

Differential Revision: https://phab.getgrist.com/D2572

app/gen-server/migration/1596456522124-Prefs.ts

@@ -0,0 +1,62 @@
+import {nativeValues} from 'app/gen-server/lib/values';
+import {MigrationInterface, QueryRunner, Table} from 'typeorm';
+
+export class Prefs1596456522124 implements MigrationInterface {
+
+  public async up(queryRunner: QueryRunner): Promise<any> {
+    await queryRunner.createTable(new Table({
+      name: 'prefs',
+      columns: [
+        {
+          name: 'org_id',
+          type: 'integer',
+          isNullable: true,
+        },
+        {
+          name: 'user_id',
+          type: 'integer',
+          isNullable: true,
+        },
+        {
+          name: 'prefs',
+          type: nativeValues.jsonType
+        }
+      ],
+      foreignKeys: [
+        {
+          columnNames: ['org_id'],
+          referencedColumnNames: ['id'],
+          referencedTableName: 'orgs',
+          onDelete: 'CASCADE'  // delete pref if linked to org that is deleted
+        },
+        {
+          columnNames: ['user_id'],
+          referencedColumnNames: ['id'],
+          referencedTableName: 'users',
+          onDelete: 'CASCADE'  // delete pref if linked to user that is deleted
+        },
+      ],
+      indices: [
+        { columnNames: ['org_id', 'user_id'] },
+        { columnNames: ['user_id'] },
+      ],
+      checks: [
+        // Make sure pref refers to something, either a user or an org or both
+        {
+          columnNames: ['user_id', 'org_id'],
+          expression: 'COALESCE(user_id, org_id) IS NOT NULL'
+        },
+      ]
+    }));
+    // Having trouble convincing TypeORM to create an index on expressions.
+    // Luckily, the SQL is identical for Sqlite and Postgres:
+    await queryRunner.manager.query(
+      'CREATE UNIQUE INDEX "prefs__user_id__org_id" ON "prefs" ' +
+        '(COALESCE(user_id,0), COALESCE(org_id,0))'
+    );
+  }
+
+  public async down(queryRunner: QueryRunner): Promise<any> {
+    await queryRunner.dropTable('prefs');
+  }
+}