EZP-22655: Remove sections

Link: https://jira.ez.no/browse/EZP-22655

## Description
This PR adds the possibility to select sections in the section list and to delete them.

![sectionlist1](https://cloud.githubusercontent.com/assets/4035241/5125489/24dd465a-70c5-11e4-983c-6c1b8cc8e1ae.png)

## TODO
- [x] Test with a non admin user and remove the todo in list.html.twig
- [x] Fix submit button icon
- [x] Fix the error markup

## Tests
Manual tests
Why does it expect an array and not a `SectionList` object ?