Deduplicate prefix form and minor refactoring

The form for adding and editing a prefix used completely different
template code and as such, adding a new field would require adding it in
two places. This code is now separated into its own template
(prefix_form.html) which is included from both the prefix add and prefix
edit page. No more duplicated code!

Fixes #395.
Unify indent style across templates

Use tabs and not spaces and do it uniformly across all templates.
Merge branch '395-dedupe-prefix-forms'

Conflicts:
	nipap-www/nipapwww/templates/base.html