Add specialist sector immigration-operational-guidance

This adds a route to the specialist_sectors controller for the sector 
'immigration-operational-guidance'. I've refactored the existing route 
for oil-and-gas so the same routes are shared.

This tag already exists in the Content API, so this is not dependent on 
any other changes in order to work.
Merge pull request #533 from alphagov/add-immigration-operational-guidance

Add specialist sector immigration-operational-guidance