Add editable helpers to Authorizable

I am working on adding editing templates to the application and need
a way to check if a user can edit a field.
Add a field template tag

There are several places in the application where we need to create
in place edit forms and show and hide values and edit fields based on
user permissions. This template tag wraps up the logic of fetching
metadata about a field and passing into the context of a child template
to render the actual markup.
Import ugettext as trans

The team decided to not alias translation functions to _, which we use
for throwaway function return values. We instead settled on 'trans'
which matches the template tag for translation.
Add in-place edit to user page

Implemented using the custom 'field' template tag.

I added public_fields and private_fields to the context so that I could
translate the static labels, since django forbids nesting our custom
'field' block tag inside a 'blocktrans' tag.

I factored the route helper out of the api to allow verb-based view
mapping in all urls files.
Merge pull request #228 from jwalgran/topic/form-tag

Add a reusable form field template tag and use it to build the a user edit form