fix(groups): forces content accessibility to members_only for invisible groups

When user selects "Group members only" for group visibility, javascript sets
the group content accessibility field to "Members only" and disables it.

For existing groups the above is done also on server side.

When editing an existing public group, a warning is added under the group
content accessibility input saying that changing the setting won't change
the access permission of existing group content.
fix(embed): file embedding wasn't working for textareas

Issue was introduced in 8b358c5 by removing the line that applies the text
change in textarea.

The way to extend javascript from the rich text editor was to extend a view
in js/embed/embed. It has been refactored to the 'embed', 'editor' js hook,
in order to make js/embed/embed convertible to js file. The view extension
has been deprecated.

Text areas and rich text editors manage the insertion diferently. In order
to insert the content in the correct place, we allow rich text editors to
return false inside the hook to stop the insert, and apply it just once.

Fixes #6160
Merge pull request #6431 from juho-jaakkola/group_settings

fix(groups): forces content accessibility to members_only for invisible groups
Merge pull request #6547 from sembrestels/embed-textarea

fix(embed): file embedding wasn't working for textareas