Make document/tag/file lists more readable

Test pages: 
* https://developer.mozilla.org/en-US/docs/all
* https://developer.mozilla.org/en-US/docs/tags
* https://developer.mozilla.org/en-US/docs/tag/JavaScript
* https://developer.mozilla.org/en-US/docs/files
* https://developer.mozilla.org/en-US/docs/templates
* https://developer.mozilla.org/en-US/docs/needs-review/

Changes:

1) Pages with the slug "User_talk:" should be excluded as well (see https://developer.mozilla.org/en-US/docs/all?page=106). These are useless for us. 

2) Ordering is changed from "title" to "slug", which will allow us to better navigate through the doc structure.

3) Displaying the slug of docs/tags/files has been removed. For docs I added the truncated doc summary:
``` html 
<small>{{ doc.get_summary() | truncate(100) }}</small>
```
4) Removed the CSS as it was imho just wasting space and things look OK without it (these pages haven't been styled for the redesign probably).

Hopefully this all is a good first step towards fixing [bug 803185](https://bugzilla.mozilla.org/show_bug.cgi?id=803185 "Provide a tree visualization of all articles (MDN index)").