Improve configuration tab's style

Hello,

This pull request is a follow-up to #312. It fixes the style of configuration tabs so that they better fit the site's design. There are several commits to ease the review process and make blaming easier:

* ed89883f: Cherry-picked from #312 ; fix the size of the tabs to address #310.

* 6d0f72bf: Update jQuery UI to 1.11.2

* 0d1b9951: Make the configuration tabs fit with the style of the site. They are styled with existing colors and styles in order to be consistent.

* faa0a957: Fix the bad displaying of configuration tabs when there's not enough content between the `.conftabs` and the left navigation ; there's a bug with left-floating elements so the content of the tab go under the navigation because the container [lose it's height](http://stackoverflow.com/a/7476799/2663959).

#### Before:

![tabs-before](https://cloud.githubusercontent.com/assets/354185/5154809/845c7e20-726e-11e4-8c0a-4cda23ac4984.png)

#### After:

![tabs-after](https://cloud.githubusercontent.com/assets/354185/5154811/87add600-726e-11e4-9790-33e1615eee54.png)

Fixes #310 and #329.

Have a nice day!
Missing trailing / to close the tag