* 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

This adds the ability to filter user signup with a whitelist
instead of or in addition to a blacklist.

Fixes #817

make regeneration worker unique, (only schedule/execute once at a time)

do not re-create a dynamic class on each feed push call, make sure redis-rb uses hiredis

adjusted to allow choosing it, yet

reinsert it when someone boosts it

status is not in the system at time of distribution; fix #607 - reset privacy
settings to defaults when cancelling replies

redis-backed "mentions" timeline as redundant (given notifications)

fix ProcessFeedService pushing status into distribution if called a second time
while the first is still running (i.e. when a PuSH comes after a Salmon slap),
fix not running escape on spoiler text before emojify

Federate spoiler_text using warning attribute on <content /> instead of a <category term="spoiler" />
Clean up schema file from accidental development migrations

affected, replaced onClick handler with onMouseUp/Down to detect text
selection not trigger onClick handler then

background as well since it's a computationally expensive

now return empty hash instead of throwing a 404 error. When following,
merge into timeline will filter statuses

application website validation, don't link to app website if website isn't set,
also comment out animated boost icon from #464 until it's consistent with non-animated version

with YAML-defined defaults. Add admin page for editing global settings. Add "site_description"
setting that would show as a paragraph on the frontpage

resolving the confusion from self-chain replies ultimately linking to a
non-self status. Adjust filters

Reflect "requested" relationship in API and UI
Reflect inability of private posts to be reblogged in the UI
Disable Webfinger for locked accounts