Dropdown menu for adding prefixes on prefix list-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.

Fixes to a few smaller issues found along the way are added as well...
Here I used inlined templates. Perhaps confusing?