Make session data delete possible with active_record_store

Conflicts:
	db/schema.rb
Correct `#user .logged-in` ids and icons

* The env admin link and the user pannel had the same controlpanel id.
* The user icon is now recovered.
* The #manage-enterprises icons was croped.

Closes ActionItem2817
finishing action item 2722
Finishing unit test action item 2722
Fixes the layout of nested comments on the wall

  - Makes the hierachy clearer with padding.
  - Removes a white border that was giving the impression of broken
  layout.

(ActionItem2726)
(ActionItem2725)
Adds user icon for null avatar on user menu

That commit makes gravatar helper to work on this context.
custom_forms: return to profile main page when cancelling submission

AI2870
uploaded-file: don't display private images thumbnails and private images on slideshow

AI2824
Merge commit 'refs/merge-requests/376' of git://gitorious.org/noosfero/noosfero into merge-requests/376

Conflicts:
	db/schema.rb
Merge commit 'refs/merge-requests/392' of git://gitorious.org/noosfero/noosfero into merge-requests/392
erge commit 'refs/merge-requests/390' of git://gitorious.org/noosfero/noosfero into merge-requests/390
Merge commit 'refs/merge-requests/387' of git://gitorious.org/noosfero/noosfero into merge-requests/387

Conflicts:
	app/helpers/application_helper.rb
	public/designs/themes/base/style.css
Merge commit 'refs/merge-requests/425' of git://gitorious.org/noosfero/noosfero into merge-requests/425
Merge commit 'refs/merge-requests/407' of git://gitorious.org/noosfero/noosfero into merge-requests/407
Fix: updated db/schema
Merge branch 'stable' of https://gitlab.com/noosfero/noosfero into stable
application-helper: refactor gravatar logic
Do not duplicate filters for plugins
Do not build view_paths on each request

Add plugin view paths at initialization.
Calling prepend_view_path on each request is too slow.

Do not include theme dir at view path.
Themes must use theme_include method to render rhtml files (instead of using render directly).
This avoids conflict of files with same name at different themes and reduces the amount of paths in view_paths.
pending-tasks-counter: add css preference

Also increasing margin by 1px.