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).
In layout.less:
```
#xwikimaincontainer {
  .container-fluid();
}
```

So yes, it is inserted as a higher level.