TASKMAN-17: Improve task view for Flamingo

This a part of https://github.com/xwiki-contrib/application-task/pull/5 by @maximaker about [TASKMAN-17](http://jira.xwiki.org/browse/TASKMAN-17).  Another pull request is coming for [TASKMAN-18](http://jira.xwiki.org/browse/TASKMAN-18).
I wonder if I need that?  @gdelhumeau, is the container inserted as a higher level of the DOM of a wiki page?  Because considering the documentation on [containers](http://getbootstrap.com/css/#overview-mobile) of Bootstrap, they are not nestable.