Check pagination wrt new pastes

[amcnamara]

Reminder to self: verify this is the case.

While I was bouncing through model/paste I noticed that the pagination offset is tied to the most recent paste instead of whatever it was when the user's session first entered the all-pastes view; in which case if 20 pastes were added since the window rendered hitting next would fetch the same 20 pastes...

[causal-agent]

One way to solve this would be to paginate by using "pastes after" an id, rather than page number.