* Rewrite Atom generation from stream entries to use Ox instead of Nokogiri::Builder

StreamEntry is now limited to only statuses, which allows some optimization. Removed
extra queries on AccountsController#show. AtomSerializer instead of AtomBuilderHelper
used in AccountsController#show, StreamEntriesController#show, StreamEntryRenderer
and PubSubHubbub::DistributionWorker

PubSubHubbub::DistributionWorker moves n+1 DomainBlock query to PubSubHubbub::DeliveryWorker
instead.

All Salmon slaps that aren't based on StreamEntry still use AtomBuilderHelper and Nokogiri

* All Salmon slaps now use Ox instead of Nokogiri. No touch from status on account

make status pagination headers generation more lax about next page
existing

in case of debugging incoming salmons. Add <thr:in-reply-to /> to favourite
salmons because it seems to be required by GS.

StreamEntry model. Simply render Salmon slaps as they are needed

background as well since it's a computationally expensive

skip ActionCable for follow/unfollow/block events, instead clear UI from
blocked account's posts instantly if block request succeeds. Add forgotten
i18n for sensitive content

timeline reload in UI, other small fixes

behaviour with Turbolinks enabled, optimizing Rabl for production