The Nginx configurations used by Nanobox previously neglected to increase the default upload size limit. This PR bumps that value up to the current Mastodon limit of 8MiB.

* Nanobox Support

- Added support for running Mastodon using Nanobox, both for local development, and for deployment to production
- Dev mode tested and is working properly
- Deployment is undergoing test as of this writing. If it works, this line will be amended to state success; if not, one or more subsequent commits will provide fixes.

* [nanobox] Resolve Deploy Issues

Everything seems to work except routing to the streaming API. Will investigate with the Nanobox staff and make fix commits if needed.

Changes made:
- Also need `NODE_ENV` in production
- Node runs on `:4000`
- Use `envsubst` to commit `.env.production` values, since `dotEnv` packages don't always support referencing other variables
- Can't precompile assets after `transform` hook, but do this locally so it only has to be done once.
- Rails won't create `production.log` on its own, so we do this ourselves.
- Some `start` commands run from `/data/` for some reason, so use absolute paths in command arguments

* [nanobox] Update Ruby version

* [nanobox] Fix db.rake Ruby code style issues

* [nanobox] Minor Fixes

Some minor adjustments to improve functionality:

- Fixed routing to `web.stream` instances
- Adjust `.env.nanobox` to properly generate a default `SMTP_FROM_ADDRESS` via `envsubst`
- Update Nginx configs to properly support the needed HTTP version and headers for proper functionality (the streaming API doesn't work without some of these settings in place)

* [nanobox] Move usage info to docs repo

* [nanobox] Updates for 1.2.x

- Need to leave out `pkg-config` since Nanobox deploys without Ruby's headers - create a gem group to exclude the gem during Nanobox installs, but allow it to remain part of the default set otherwise
- Update cron jobs to cover new/updated Rake tasks
- Update `.env.nanobox` to include latest defaults and additions

* [nanobox] Fix for nokogumbo, added in 1.3.x

Apparently, nokogumbo (pulled in by sanitize, added with `OEmbed Support for PreviewCard` (#2337) - 88725d6) tries to install before nokogiri, despite needing nokogiri available to build properly. Instruct it to use the same settings as nokogiri does when building nokogiri directly, instead of via bundler.

* [nanobox] Set NODE_ENV during asset compile

The switch to WebPack will rely on the local value of the NODE_ENV evar, so set it to production during asset compilation.

* [nanobox] Rebase on master; update Nginx configs

- `pkg-config` Gem no longer causes issues in Nanobox, so revert the Gemfile change which allowed excluding it
- Update Nginx configuration files with latest recommendations from production documentation
- Rebase on master to Get This Merged™

Everything should be golden!