default values for extlink JS settings not working.

Problem/Motivation

The D7 version of extlink provides text alternatives for the external link icons, which are important for screen readers. The icon itself is a CSS background image, and the text alternative is provided in a span.element-invisible element.

This has stopped working in the D8 version of the module. The expected <span> element is present, however:

• It has no text content, i.e. the alternative text itself is missing
• It still uses the D7 element-invisible class. This has been renamed to visually-hidden in D8.

The problem appears to be incorrect use of $config->get() during hook_page_attachments(). The module is trying to provide default values here, like the D7 variable_get(), but in D8 values are always expected to come from the config store.

Additionally, the installation config and config schema don't include all the settings keys that hook_page_attachments() is trying to use. Neither of these config files mention extlink_label, so it doesn't have a value after installing the module. Hence the JS fails to write the text alternative for the icon.

So overall, this looks like a few steps were omitted when porting the module to D8.

Steps to reproduce

• Install the D8 version of External Link module, and enable the "place an icon..." settings on the module's administration page.
• Create a node which has a link in the body text, with a URL to an external site.
• View the node. The extenal link has an icon after it. This is expected, but...
• A screen reader will read the link text, but there is no indication that the link is external.
• Inspect the link using browser dev tools. The link has a descendant <span class="element-invisible"> but it has no text content. This is where the JS is supposed to put the text alternative.
• Inspect the DOM using browser dev tools. There is a drupalSettings.data.extlink object, but the extLabel property is an empty string.

Compare this with the D7 version, after the same steps:

• The link contains <span class="element-invisible">(link is external)</span>
• This text comes from Drupal.settings.extlink.extLabel javascript object.
• A screen reader will include this text in the link name.

Proposed resolution

Complete the module's config (install and schema) so that it provides all the default values needed.

Remaining tasks

• Change element-invisible class to visually-hidden in the JS file. This class changed name in D8 core CSS.
• Make the same change in the extlink.min.js file
• Complete the module's config schema (config/schema/extlink.setting.yml) so that it describes all of the config keys accessed by extlink_page_attachments() in the module file.
• Move the installation config (config/extlink.setting.yml) to the correct location: config/<strong>install</strong>/extlink.settings.yml.
• Update the installation config (config/install/extlink.settings.js)) so that it has all the default values currently described in extlink_page_attachments().
• In extlink_page_attachements(), remove the second argument from every calll to $config->get(). This method only accepts one argument in D8.
• [possibly...?] Write an update hook so existing installations get the text alternatives for the icon.

User interface changes

Fix markup for the external link icon so it includes a text alternative, like the D7 version of the module does.

API changes

None.

Data model changes

Completes the config schema so it has all the settings which the module makes use of.
Updates the installation config so default values are provided.

Original report by naveenvalecha

Original issue title: Screen reader unable to read out External Links

Steps to Reproduce:
1. Create any external link by creating any node.
2. Enable the Screenreader
3. Navigate to External link using keyboard

*Observation:* Screen reader is unable to read it as an external link.
Patch the external link module for accessibility.

We can have workaround like this site has.