www: Pool selector dir. now uses scope binding

Modified the pool selector directive to use scope binding to communicate
what pool has been selected. This adds some flexibility at we can choose
what variable in the parent scope to assign the selected pool to,
instead of letting the directive assign directly to $parent.from_pool.
www: Three small visual corrections

Three small fixes to the web UI:
* Avoid breaking lines in the cogwheel menu
* Avoid breaking lines whn displaying prefix lengths in pool list
* Change title of prefix cogwheel menu to "Prefix" to match the others
www: Link to new add prefix page

Replaced the "Add prefix"-button on the prefix list page with an
AngularJS/Bootstrap dropdown menu with three options: Manual, From pool
and From prefix.

When choosing From pool or From prefix the user gets a message
explaining that the operations now are performed from the pool list and
prefix list pages respectively.
www: add button for from-pool prefix creation

Since the most common operation involving a pool is adding new prefixes
from a pool we add a nice green button for this so it's easy to find
instead of hiding it away in our cog wheel menu.
Merge branch 'garberg-link_to_ng-prefix-add'
Add boilerplate for pynipap
www: add expires attribute to xhr
www: add datepicker to prefix form
www: add logic for expires field
www: add AVPs for VRFs

This adds the ability to add, modify and remove AVPs for VRFs.
Implemented using AngularJS - my first Angular code, yay! =)

Fixes #637.
www: fix CSS for datepicker

The .listing class was inadvertently applied to more tables than
intended.

.listing th, td {
    ...
}

Means th element with class listing and all td elements, not just td
elements with class listing.
Merge pull request #656 from plajjan/637-add-web-avps-for-vrfs

www: add AVPs for VRFs
Merge pull request #658 from plajjan/599-web-expires

Add expires field to web UI
www: add AVPs for prefixes

This adds the ability to add, modify and remove AVPs for prefixes.

Fixes #636