Refactor/css rename

In accordance with: https://groups.google.com/forum/#!topic/light-table-discussion/D6-aqBIfPFQ

* Renames #side -> #left-bar
* Renames #bottombar -> #bottom-bar
* Renames #statusbar-container -> #status-bar
  - This has weird ramifications since #statusbar-container was implemented differently than the other containers. In most cases, containers were directly named and contained an internal .content div, e.g.
```html
  <div id="left-bar">
    <div class=".content">
      ...
    </div>
  </div>
```
  I suggest that we also rename this to match before merging this PR.

Note:
This has direct ramifications for skin plugins, but should not impact themes. My personal skin, lightshow, was minimally effected since I decoupled plugins and widgets from their containers, so e.g. the workspace was still styled correctly even though the #side identifier had become #left-bar.

Also note:
clojure symbols / filenames now have less bearing on the actual dom ID which is kind of unpleasant. I'd like to see those updated as well, but that will be a significantly more involved change, and I think is safe to separate and do at a later date.