* Add additional documentation and warnings to the WEB_DOMAIN setting.

This feature is largely undocumented, and quite a number of users have
shot them in the feet already despite the warning. Added a bit of documentation
and expanded the warning until we have a mechanism for dealing with conflicting
user URIs.

* Change WEB_DOMAIN comments to point to the extensive online documentation

connection options instead of setting REDIS_HOST etc individually

Close #1986

Without setting it, exim will reject the mail with a message like:
rejected EHLO from [10.20.0.1]: syntactically invalid argument(s): {}

* Allow using an SMTP server without authentication (e.g Postfix relay on the same host) by setting SMTP_LOGIN and SMTP_AUTH_METHOD to 'none'

* Add note in .env.production.sample about SMTP settings for servers where no auth is required

* Assume that SMTP_LOGIN and SMTP_PASSWORD will be blank if we set SMTP_AUTH_METHOD to none

* Allow running mastodon on a different domain as the one used for identifying users

* Alter documentation of WEB_DOMAIN to make clear it shouldn't be used unless the admin knows what they are doing

* Compare to web_domain instead of local_domain when dealing with feeds/API

* Correctly identify mentions to local accounts

Mentions URLs point to the person's web profile, i.e., the user page served on WEB_DOMAIN.

* Custom Paperclip path.

* Document PAPERCLIP_ROOT.

* Add PAPERCLIP_ROOT_URL (and rename PAPERCLIP_ROOT to PAPERCLIP_ROOT_PATH).

* delivery fix

# Conflicts:
#	config/environments/production.rb

* added stub in .env file

* reordered and added a comment

* Allow SMTP auth method customization

* Add SMTP openssl_verify_mode option support

Allows one use self-signed certs with their SMTP server.

* Add SMTP enable_starttls_auto option support

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

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

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

Fixes #817

add support for reading S3 protocol from ENV
also add S3_HOSTNAME in .env.production.sample

Make the streaming API also handle websockets (because trying to get the browser EventSource interface to
work flawlessly was a nightmare). WARNING: This commit makes the web UI connect to the streaming API instead
of ActionCable like before. This means that if you are upgrading, you should set that up beforehand.