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'