www: Added AngularJS version of add pool-page

Added an AngularJS version of the add pool-page. Templates prepared for
reuse on edit pool-page.
www: Added ngified basic pool edit functionality

Added an AngularJS implementation of basic pool edit functions and
statistics. What's missing is adding prefixes to a pool.
www: Added pool select directive to prefix forms

Added a directive showing a popup where a pool can be selected. This was
added to the prefix add and edit forms, so now the pool a prefix belongs
to can be set directly on the prefix.

During the work with the directive a bug was discovered. The bug was
introduced when the prefix form definitions were moved to a separate
template included with ngInclude. ngInclude created a new scope, which
means that the $scope properties which are bound to the VRF and pool
selector directives are bound to ngInclude's scope and not the
controller's scope.

To fix this the VRF and pool variables were moved from the scope to the
prefix object.
www: Add explanation of how pools are expanded

Added a popup-window triggered by the "Expand pool"-button on the pool
edit page, ewxplaining to the user how pools are expanded since version
0.27.
www: Link to angularified edit pool page

Changed the links on the pool list page to point towards the
angularified pool edit page.

Fixes #638
Merge pull request #659 from garberg/638-angularify_pool

angularify pool add/edit page
www: New image of how prefs are added from pools

Updated the screenshot showing how prefixes are added from pools to
match the updated workflow.