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.

![sectionlist](https://cloud.githubusercontent.com/assets/4035241/5089188/e5775014-6f39-11e4-8e56-86d9ef2e84d7.png)


## TODO
- [x] Test with a non admin user and remove the todo in list.html.twig
- [x] Fix submit button icon

## Tests
Manual tests
same remark as on the previous `form_errors` call (btw are both needed?), does this produce a correct markup ?