Modified add prefix page for use with bootstrap

Changed the "tooltip"-class used to trigger the tipTip module to
"tiptip". The tooltips does currently not work on the AngularJS-version
of the add prefix page though, due to the tiptip module being activated
before Angular has rendered the view.
Improved handling of pools without default preflen

Improved how assigning prefixes from pools without default prefix length
for the address family in question is handled. The option to use the
default prefix length is disabled and an asterisk denoting that the
prefix length field is required was added.

Fixes #445