Adding json response for 410 when triggered with extension .json

[ssadhu-sl]

Reference ticket: https://trello.com/c/MNP2L46O/1405-bug-410-responses

• Returns status code 410 instead of 200 for 'Entity removed' page
• Added case for returning json response for 410s when called with .json extension
• Changed the html page content/design for 410 to match other error pages
• Updated test cases to reflect the changes

[CarlosCoelhoSL]

lgtm just one small thing :)

[eveleighoj]

Did you need to write a test to check that 404 are being raised (when you showed your screen it was giving a 500 error)?