Refactoring the default theme (for great justice)

So the current state of the default theme is terrible. The CSS is a mess, the HTML is a mess, and it was all hacked together when I was just trying to get a MVP running.

Things are different now. We have the technology to make this a lot better, so that's what this PR is set out to do. Here's what I'm planning:

* [ ] Rewrite all of the default theme templates using [skeleton.css](http://www.getskeleton.com/). (These subtasks are somewhat ranked by priority)
    * [x] `base.html`
    * [x] `index.html`
    * [x] `board.html`
    * [x] `thread.html`
    * [x] `post_editor.html`
    * [x] `user.html`
    * [x] `user_settings.html`
    * [x] `login.html`
    * [x] `register.html`
    * [ ] `admin.html`
    * [ ] `admin_users.html`
    * [ ] `admin_user.html`
    * [ ] `admin_boards.html`
    * [ ] `action_move_thread.html`
* [x] Redo all of the CSS in SASS, so it's remotely reasonable to work with/maintain.
* [x] Make the original theme easier to use/overall less jank as fuck.
* [x] Make the site responsive, so it doesn't look like donkey shit on phones and tablets.

:beer: