Affix the sidebar

This fixes the sidebar so that it does not scroll with the log.

![drone-affix-sidebar](https://cloud.githubusercontent.com/assets/103167/4798510/7c6db04e-5e14-11e4-9bc4-78a90175751f.png)

The diff looks weird in GitHub (turning off whitespace [cleans it up](https://github.com/drone/drone/pull/636/files?w=1)), but the only new tag is the `<div id="sidebar-inner">` wrapping everything inside `<aside id="sidebar">`. I don't think there is a way to use `position: fixed` and the `#main { flex-grow: 2 }` rules together without creating this new tag.

If you'd like any semantics different from `id=sidebar-inner`, etc. let me know.