Allow setting of default language through config

Setting of locale in controller extracted to Localized concern,
the doorkeeper authorized applications controller moved under
custom namespace with inclusion of Localized, which resolves the
"it sometimes appears in a different random language" bug

* 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

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

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

Use nicer scrollbars in MS edge

Corrected spelling error for "install"