Follow-up to #8142

* Redesign landing page (again)

* Move login form in small version to the right column

* Display closed registrations message

* Add site setting for the hero image

* Fix test

* Increase spacing, maximum width, change call to action section

They were redundant, and also had a inconsistency; the button for
"other instances" had an icon for the external link in "more" page, but
it didn't in the other pages.

This unifies the implementation, and the external link icon is now shown
in all the about pages.

* Improve OpenGraph tags for about pages

* Add thumbnail admin setting

* Fix error

* Fix up

* Add Mastodon::Source.url

* Update spec

* Refactor

Move things frmo Mastodon::Source to Mastodon::Version

apply same style class to the About description on both the landing page
and the about/more page

* Redesign extended information page

* Remove unused keys

* Add $ui-base-lighter-color variable to SCSS

* add haml_lint to Gemfile

* add .haml-lint.yml

* fix warnings of haml_lint

* Adds version.

* Cleans up code.

* Removes standalone endpoint and adds version to instance endpoint.

* Addresses feedback from #2181.

* Simplify render in mailer views

* Simplify render in about/more view

* Simplify render in accounts follow views

* Simplify render in recovery code views

* Simplify render in stream entry embed

* Simplify render in stream entries status partial

* Simplify render in authorize follow new view

* Move site title helper to instance helper (name change only)

* Remove newline in <title> tag

* Add site_hostname helper method to wrap up local_domain value

* Use site_hostname helper in places that need local_domain value

* Correct site_contact_email typo

* Separate about more page into partials, add specs

* Add InstancePresenter to expose site details

* Clean up about controller, use instance presenter