Refactor and rename `restricted_signup_domains` to `domain_whitelist` to better conform to its behavior and newly introduced behavior.

This reverts commit 3e991230.

# Conflicts:
#	app/models/project.rb

stub Devise.omniauth_providers to return GitHub even if the gitlab.yml has no omniauth provider enabled
This will fix failing tests in case gitlab.yml file has no omniauth providers enabled

In 8278b763 the default behaviour of annotation
has changes, which was causing a lot of noise in diffs. We decided in #17382
that it is better to get rid of the whole annotate gem, and instead let people
look at schema.rb for the columns in a table.

Fixes: #17382

I’ve removed everything related to the feature based on this commit:
ce08f919f34fd8849834365

Resolves #10857.

Also:
- Get rid of legacy :strict_mode
- Get rid of custom :email validator
- Add some shared examples to spec emails validation

`max_attachment_size` in `ApplicationSetting` should be present,
only integers greater than zero are valid.

Closes #13188

Signed-off-by: NDmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

delay is in seconds
more legible code in session_store
Added `GitLab restart required` help block to session_expire_delay

settings

This feature was requested long ago:

http://feedback.gitlab.com/forums/176466-general/suggestions/4118466-ability-to-register-only-from-ceratain-domains

This MR is based off !253 but changed to use application settings and use wildcard strings
to give more flexibility in pattern matching. Regexps seemed overkill and easy to get wrong.

Only restrict e-mail addresses upon creation

Add checkboxes to the application settings page for restricted
visibility levels, and remove those settings from gitlab.yml.

Signed-off-by: NJeroen van Baarsen <jeroenvanbaarsen@gmail.com>