* Increase DNS timeout from 1 second to 5 seconds for MX check

1 seconds is rather short when using a recursive DNS resolver which
hasn't got a cached result already available. Use 5 seconds instead,
which is the timeout value we use for outgoing HTTP queries.

* Add more precise error messages for invalid e-mail addresses

* Change e-mail whitelist/blacklist to not be checked when invited

And only when creating an account, not when updating it later

Fix #10648

* Fix test

* Add specs for BlackListedEmailValidator

* Use instance variable

* Implement BlacklistedEmailDomain

* Use Faker::Internet.domain_name

* Remove note column

* Add frozen_string_literal comment

* Delete unnecessary codes

* Sort alphabetically

* Change of wording

* Rename BlacklistedEmailDomain to EmailDomainBlock

* Stricter whitelist rules

* Linting

* Added spec for blacklisting

* Test subdomain blacklist on domain whitelist

* No need to split

* Change spec name

* Add `Presenters` group to SimpleCov configuration

* Move validators to app/validators, add to simplecov config

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

Fixes #817