Allow DB downgrade

app/src/main/java/eu/faircode/email/DB.java

@@ -3025,6 +3025,11 @@ public abstract class DB extends RoomDatabase {
                         logMigration(startVersion, endVersion);
                         db.execSQL("UPDATE `account` SET `prefix` = NULL");
                     }
+                }).addMigrations(new Migration(297, 296) {
+                    @Override
+                    public void migrate(@NonNull SupportSQLiteDatabase db) {
+                        logMigration(startVersion, endVersion);
+                    }
                 }).addMigrations(new Migration(998, 999) {
                     @Override
                     public void migrate(@NonNull SupportSQLiteDatabase db) {