Controller actions and access control for sequences

[#51592849]

https://www.pivotaltracker.com/story/show/51592849
Sequences have owners

https://www.pivotaltracker.com/story/show/51592849
Adds thumbnail_url to activities [#51592849]

https://www.pivotaltracker.com/story/show/51592849
Make view specs work for sequences
HAMLizing ERb views