Make database structs mutable.
Make some changes to ensure smooth transition for the next commit.

Update to this one first in order to migrate the database.
Remove temporary code & make changes to support the new layout.

  * Since `Record`s are mutable, `Thread.skipped` is now an `int`, not an `IntRef`.
    Thus, `.skipped.value` => `.skipped`.
  * `Post`s have a new field, `ip`. Accordingly, `thread_create` and `post_create`
    require one more argument to fill that field with, which `viewserver.html`
    provides. Don't forget to configure any proxies so that the real IP is
    forwarded as X-Forwarded-For.
  * `Thread`s have two new fields, `attached` and `closed`, which do nothing
    now and cannot be changed from their default values of `False`.

Also, since `Record`s are not sortable, sorting them by attribute `id` is required.