support other SQLite wrappers, and various hooks needed by grist-static (#516)

app/gen-server/entity/Secret.ts

@@ -3,10 +3,10 @@ import {Document} from "./Document";
 
 @Entity({name: 'secrets'})
 export class Secret extends BaseEntity {
-  @PrimaryColumn()
+  @PrimaryColumn({type: String})
   public id: string;  // generally a UUID
 
-  @Column({name: 'value'})
+  @Column({name: 'value', type: String})
   public value: string;
 
   @ManyToOne(_type => Document, { onDelete: 'CASCADE' })