Add "recordings" table, save raw audio blobs in it

Other changes:
- Move Stored(Voicemail|Recording) interfaces into knex/types/tables
module for reduced boilerplate.
- Change updateStoredVoicemail to take Partial<StoredVoicemail>,
allowing to only update only some columns

src/db/migrations/20210311061107_create_audio_table.ts

@@ -0,0 +1,13 @@
+import { Knex } from "knex";
+
+export async function up(knex: Knex) {
+  await knex.schema.createTable("recordings", (table) => {
+    table.bigInteger("messageId").primary().references("voicemails.messageId");
+    table.string("mimeType", 32);
+    table.binary("audio");
+  });
+}
+
+export async function down(knex: Knex) {
+  await knex.schema.dropTable("recordings");
+}