(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/entity/Pref.ts

@@ -0,0 +1,31 @@
+import {Prefs} from 'app/common/Prefs';
+import {Organization} from 'app/gen-server/entity/Organization';
+import {User} from 'app/gen-server/entity/User';
+import {nativeValues} from 'app/gen-server/lib/values';
+import {Column, Entity, JoinColumn, ManyToOne, PrimaryColumn} from 'typeorm';
+
+@Entity({name: 'prefs'})
+export class Pref {
+  // This table may refer to users and/or orgs.
+  // We pretend userId/orgId are the primary key since TypeORM insists on having
+  // one, but we haven't marked them as so in the DB since the SQL standard frowns
+  // on nullable primary keys (and Postgres doesn't support them).  We could add
+  // another primary key, but we don't actually need one.
+  @PrimaryColumn({name: 'user_id'})
+  public userId: number|null;
+
+  @PrimaryColumn({name: 'org_id'})
+  public orgId: number|null;
+
+  @ManyToOne(type => User)
+  @JoinColumn({name: 'user_id'})
+  public user?: User;
+
+  @ManyToOne(type => Organization)
+  @JoinColumn({name: 'org_id'})
+  public org?: Organization;
+
+  // Finally, the actual preferences, in JSON.
+  @Column({type: nativeValues.jsonEntityType})
+  public prefs: Prefs;
+}