Allow to limit content access per channel - edit channel

Background

Content access can currently be set in the workflow. It is very flexible in status, but when a content type is shared between channels the access can't be limited to a channel.

Implementation

To feature this a "Read access" and "Write access" will be added:

• Multiple users groups can be added to a channel
• When no groups have been added, everyone has access

Relation to other issues

• The "Read access" and "Write access" settings will be used for website management, block management, etc. in a later issue, so that's why it is not limited to content.

Visual

(Please note that the text in the screenshot is wrong, should be "Read access" etc.)

Requirements

• Add a "Read access" and "Write access" setting to the /admin/channel/new and edit page. The placeholder is "Everyone". The component is a multi select2 for group selection, same as for the workflow
• Add the "Everyone" placeholder to the workflow edit page as well