bring back ie6 and ie7 css - use asset pipeline for them

We now have two ways of processing stylesheets.
The stylesheets living in app/stylesheets are processed with our own theming
The stylesheets living in app/assets/stylesheets use the asset pipeline
also compile assets with the asset pipeline
prefix ui/person_dark.png with /images

this avoids using the asset pipeline.

We will probably want to use the asset pipeline for images as well at some point.
But for now i just work around the error that occured in production when the
image could not be found in the precompiled assets.
properly deploy with default asset precompile
do not inherit Context::User in Context::Me

otherwise both tabs will be active in the ui. Inheritance was used for the model_name
class method. Now inheriting this from Context and setting the wrapped_base_class on
each inherited class. This also removes duplication between Context::User and
Context::Group
add a test for #6757 - looks like it's fixed
fix context_pages_controller with tests
Merge pull request #65 from azul/bugfix/two-active-tabs

do not inherit Context::User in Context::Me
Merge pull request #64 from azul/bugfix/context-page-urls

fix context_pages_controller with tests
cache me_menu based on theme

the theme contains the navigation
hide hidden groups just like hidden users (#6755)
Merge pull request #48 from azul/bugfix/visibility-fixes-6757-6755

Bugfix/visibility fixes #6757 and test #6755
integration test for top navigation
Merge pull request #66 from azul/bugfix/topmenu-nav

cache me_menu based on theme
test for sharing during page_creation - covers #2776
Merge pull request #67 from azul/bugfix/permission_issues

test for sharing during page_creation - covers #2776
fix and test sharing pages
fix and test assigning tasks - fixes #7867
fix saving the page visit timestamp - fixes 7883

also use it for the (new) or (modified) indicators in the task page
no more need to backport from rails 3.1 :)