Add concern folder for controller generalizations

Rails 4 introduces Concerns. I use the name and the directory to add
module code. For instance, many of the controllers need to load the
repositories. I have added the code into such a concern. This way I
can add the functionality via include.

Signed-off-by: Konrad Reiche <konrad.reiche@gmail.com>
Rename concerns module

I have changed the module depth for the concerns modules. Using the
module concern followed by submodules.

Signed-off-by: Konrad Reiche <konrad.reiche@gmail.com>
Add repository selector with caret plus style

Instead of a select box I will use a link and a dropdown menu for
selecting repositories. For this I am using a header link as well
as a caret to mark it.

Signed-off-by: Konrad Reiche <konrad.reiche@gmail.com>
Update gems

Just a shot update, I was curious if Twitter Bootstrap 3 is already
available via the gem. Apparently, it is not.

Signed-off-by: Konrad Reiche <konrad.reiche@gmail.com>
Style repository selector for admin control panel

I have styled the repository selector for the admin control panel. For
this I needed to use the btn-toolbar class, otherwhise it was not
possible to center the dropdown menu with letting the menu items
disappear.

Signed-off-by: Konrad Reiche <konrad.reiche@gmail.com>