US #50: Navigate between US/Issue neighbors

A new service `SelectedTags` has been created through which you can
access the Backlog/Issues selected tags:

SelectedTags.backlog
SelectedTags.issues.tags
SelectedTags.issues.type
SelectedTags.issues.status
SelectedTags.issues.severity
SelectedTags.issues.priority
SelectedTags.issues.owner
SelectedTags.issues.assigned_to
SelectedTags.issues_order

The selection is saved also in browser's localstorage in order to
maintain the same filtering/ordering criteria when switching between
list and detail views.

A new directive `gmIssuesSortedBy` has been created in order to maintain
the "visual feedback" of the issues-listing order criteria when
switching back and forward between other views.

In the UserStory/Issue details two new buttons are displayed:
* Next - hidden if there's no other item following the current one.
* Previous - hidden if there's no other item preceding the current one.