Create a default icon component for extensions with no icon

Created by: AlicjaSuska

Part of the tracking issue #16165, RFC 250

Description

Some extensions do not have an icon assigned to them. We need to create a default icon that can be displayed in the action items bar.

Design details Figma

Icon should:

• include the first letter of the extension name
• use one of 5 colors: $oc-grape-7, $oc-violet-7, $oc-cyan-9, $oc-indigo-7, $oc-pink-8
• diabled icon: $oc-grey-5, $text-muted (light)