Initial attempt at tracking "sectors".

This commit adds the ability to track what sector(s) a top-level object
applies to. The list of available sectors is based off of the DHS
Critical Infrastructure Sectors list.

It is another field on the "Details" page for all top-level objects. It
uses the tagit jquery library (which we use for bucket lists and
campaign aliases). The list of available options is pre-populated to
choose from and drops down on field focus.

This also adds in sector count tracking similar to bucket lists as well
as searching.
Fix sector search from sector link.

The search by clicking on the link of the sector you add now functions
properly.

Also now prevents people from adding sectors which are not in the
availableTags list.
Adding UPDATING entry.

People will need to know how to add sectors to the database, so it will
tell them how.
Docstrings and bucket list autocompletion.

Added docstrings for some functions I missed. While I was here I quickly
added autocompletion for bucket list items. It will search the
"bucket_lists" collection for existing buckets as you type (minimum of
two characters) and find any that start with that string (case
insensitive). It will return that list as available autocomplete options
to the user in the UI.
Merge pull request #124 from crits/sectors

Sectors