Issue #2840909 by vishalkhialani, slv_: Replacing t() calls with $this->t() (patch for coding standards)

Refactored ViewportPermissionsExistTest into an actually useful test class to test access control to viewport settings page

Amended description on settings page to mention forward slash requirement, and replace legacy examples with new ones

Fixed testViewportTagIsPrintedWithTheRightValues(), and added testViewportTagNotPrintedInNotSelected()

Moved ViewportPermissionsExistTest to 'tests/src/Functional' and make it extend BrowserTestBase instead of WebTestBase

Replaced hook_html_head_alter() with hook_page_attachments_alter(), and updated viewport_path_is_selected() code so that it works again. D8 version back to life \o/.

Upgraded permissions by moving them to viewport.permissions,yml. Some more interface text improvements as per https://www.drupal.org/node/604342.

Moved viewport settings page to 'admin/appearance/settings/viewport' and created a menu local task for it

Adapted .info file to remove version entry, and .routing file to include title for the settings page

Adapted ViewportSettingsForm class to make proper usage of FormStateInterface , configFactory() method, and Element::children() helper

added some sensible defaults to viewport.settings.yml, finished ViewportSettingsForm Class, completed module's functionality,and added help texts through hook_menu. Could consider this a complete port, but needs some tests

added routing.yml file, settings page controller, implemented hook_menu and hook_permission, and updated info.yml file. Submit handler on form controller to be completed