fixing failing specs after refactoring Groups#memberships_for_member_list.
Introducing what's-new box on the intranet starpage.
fixing failing spec after introducing the what's-new box.

The ‚help‘ page is listed in the what’s-new box in the spec. Therefore,
this link had been ambiguous.
new model caching: refactoring Structureable.

In `StructureableMixins::Roles`, now implement the `fill_cache` and the
`delete_cache` method. Using the Ruby-2 `prepend` mechanism, the
`super` method can be called within these module methods.
Applying the new model caching to the Group model.

Trello:
https://trello.com/c/V6polddr/660-neues-konzept-fur-model-caching
applying the new model caching to Structureable.

trello: https://trello.com/c/V6polddr/660-neues-konzept-fur-model-caching
hot fix: temporarily fixing StructureableMixins#fill_cache.
adding further exception handling for caching task.
rescue from caching errors.
Trying to fix the travis issue
"Gem::RemoteFetcher::FetchError: Errno::ETIMEDOUT: Connection timed out - connect(2) for "rubygems.org" port 443"

Resources:
* https://gist.github.com/delano/845643
* https://github.com/elgalu/crafting_rails4_apps/blob/ecc0eed/.travis.yml#L13-L14
* http://blog.travis-ci.com/2013-05-20-network-timeouts-build-retries/