Cleanup old constant references

Now that the code that renamed and reworked these workers has been deployed and
the queue has had adequate time to run down, we can get rid of these temporary
references to the old workers.
Create new featured link table
Update models and unit tests for featured links

Remove top_tasks and featured_services_and_guidance and replace it with
just featured_links
Update references to top_tasks to featured_links
Add homepage types to organisations

Automatically set the type based on if there are any current featured
service and guidance link for a given organisation
More featured link and featured service guidance updates
Add new limits for featured links
Show correct number of featured links on org pages

Note: we are now only showing 7 featured service and guidance links
rather than 10. This is because in user research fewer links here have
proved to be more effective. This might be iterated in the future
though.
Update admin to show featured links
Add new feature link admin toggle
Update tests
Update permisions for managing org homepages
Add future migration to drop extra tables

These will no longer be used as the models have been deleted. Put in
pending_migration_cleanups so there are no errors while the code hasn't
restarted but the migrations have run.
Use default featured link count
Featured links are sorted by default

The association between objects and featured_links specifies an order,
so they will be sorted when they come out of the database. We don't need
to resort then in the view.
Add positive test for validity
Add some tests for new methods on an organisation
Add positive test for FeaturedLink

Also update the negative tests so the models would otherwise be valid if
it wasn't for the attribute we are testing.
Extract & reuse method to return possessive form

Extends the base String class to return the possessive form of a string and
updates the admin editions filter to use the correct form.
Update to latest poltergeist