Detailed students form and other changes

* Project title field is hidden or shown based on the type of engagement

* Organisation website field is auto-filled based on selection

* If Organisation is new, then the value in the website field is used to
  create the organisation

* Change `data.json.jbuilder` template to use new background method

* Required changes to controller action and JSON responses
  for organisation search

* Add StudentBackground model and migration

* Add migration to remove background_id field from Student model
  * Perform required data migration within the migration `change` method

* Add Student#chosen_student_background and Student#background_id
  methods

* Add migration for `approved` field for Student

* Add migration for background_order field for StudentBackground

* Add default scope to StudentBackground to order by background_order

* Fix associations
  * Student has many student backgrounds
  * Background has many student backgrounds
  * Student background belongs to student and background

* Change maintenance task to create student backgrounds relations
  for sample data

* Add search action for backgrounds

* Add student invites CRUD
  * controller
  * model
  * views
  * migration

* Fix accessor methods in concerns to mark attributes as dirty when
  changed

* New FlexibleBackground concern

* Add new fields to whitelisted params in students controller

* Add inverse associations to use objects in memory

* Fix boostrap.css; Rename to css.scss an use sprockets helpers

* ZoomIn to india level or ZoomOut to world level depending on markers

* Clean up admin menu

* Other minor changes