Add an option to append extra classes to configuration to all icons

Problem/Motivation

Translations and screen readers should ignore text in material icons.
To achieve that extra classes should be added to

Proposed resolution

Add a textfield to /admin/config/content/material_icons config page which allows to add extra classes to all icons globally.

Remaining tasks

- add text field to config form
- update configuration settings
- update twig template

User interface changes

- config settings

API changes

- none

Data model changes

- extra config settings value