* 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

Small addition in case people want email service options, sparkpost.com gives you 100k/mo free

Under ## General dependencies:
apt-get needs sudo and install was typed wrongly.

Reduce size of background-photo.jpeg

Add a couple of network performance tweaks to Vagrantfile.

Reduced by running through `guetzli` image optimizer.

Add metadata to Dockerfile

Optimize Dockerfile

Cosmetic changes to the scalingo deployment

Updates slugignore

Linux users must enable NFS for Vagrant

Just launched https://hackertribe.io/, added it to the list.

Add mastodon.land instance

Optimize Dockerfile, reduce build time.

Catch more errors in process_follows so it doesn't fail

slightly optimized FanOutOnWriteService again

Use NFS for the shared folder because it dramatically decreases latency for git operations.

Correct innacurate info and other fixes