[wip] admin backend improvements

this supercedes PR #5021 by @maxwell
for now I have kept his changes as is, but I will still implement the proposed suggestions from the original PR.

also I used the opportunity to port the admin pages to bootstrap and polish the user search in the backend a little. 
![d_admin_user_search](https://cloud.githubusercontent.com/assets/117366/3423932/403c8920-ffb3-11e3-9d1a-015e9d9424c1.png)

this is not completely finished yet, but I would love to read your feedback!
yes, the content is centered by the `%div.container`, the `%div.span-24.last` is a remnant of the blueprint 24 column layout, which I wanted to get rid of. Coincidentally bootstrap picks up on that class because of the selector `[class*="span"]`, and that definition adds a margin that shouldn't be there on one of the outermost containers of the page...